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+\begin{isabellebody}%
+\setisabellecontext{Paper}%
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+\isamarkupsection{Core of the proof%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
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+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+This paper builds on previous work by Ausaf and Urban using
+regular expression'd bit-coded derivatives to do lexing that
+is both fast and satisfies the POSIX specification.
+In their work, a bit-coded algorithm introduced by Sulzmann and Lu
+was formally verified in Isabelle, by a very clever use of
+flex function and retrieve to carefully mimic the way a value is
+built up by the injection funciton.
+
+In the previous work, Ausaf and Urban established the below equality:
+\begin{lemma}
+\isa{{\normalsize{}If\,}\ v\ {\isacharcolon}{\kern0pt}\ r{\isacharbackslash}{\kern0pt}s\ {\normalsize \,then\,}\ Some\ {\isacharparenleft}{\kern0pt}flex\ r\ id\ s\ v{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ decode\ {\isacharparenleft}{\kern0pt}retrieve\ {\isacharparenleft}{\kern0pt}r\mbox{$^\uparrow$}\mbox{$\bbslash$}s{\isacharparenright}{\kern0pt}\ v{\isacharparenright}{\kern0pt}\ r{\isachardot}{\kern0pt}}
+\end{lemma}
+
+This lemma establishes a link with the lexer without bit-codes.
+
+With it we get the correctness of bit-coded algorithm.
+\begin{lemma}
+\isa{lexer\mbox{$_b$}\ r\ s\ {\isacharequal}{\kern0pt}\ lexer\ r\ s}
+\end{lemma}
+
+However what is not certain is whether we can add simplification
+to the bit-coded algorithm, without breaking the correct lexing output.
+
+
+The reason that we do need to add a simplification phase
+after each derivative step of $\textit{blexer}$ is
+because it produces intermediate
+regular expressions that can grow exponentially.
+For example, the regular expression $(a+aa)^*$ after taking
+derivative against just 10 $a$s will have size 8192.
+
+%TODO: add figure for this?
+
+
+Therefore, we insert a simplification phase
+after each derivation step, as defined below:
+\begin{lemma}
+\isa{blexer{\isacharunderscore}{\kern0pt}simp\ r\ s\ {\isasymequiv}\ \textrm{if}\ nullable\mbox{$_b$}\ {\isacharparenleft}{\kern0pt}bders{\isacharunderscore}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r\mbox{$^\uparrow$}{\isacharparenright}{\kern0pt}\ s{\isacharparenright}{\kern0pt}\ \textrm{then}\ decode\ {\isacharparenleft}{\kern0pt}mkeps\mbox{$_b$}\ {\isacharparenleft}{\kern0pt}bders{\isacharunderscore}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r\mbox{$^\uparrow$}{\isacharparenright}{\kern0pt}\ s{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ r\ \textrm{else}\ None}
+\end{lemma}
+
+The simplification function is given as follows:
+
+\begin{center}
+ \begin{tabular}{lcl}
+ \isa{bsimp\ {\isacharparenleft}{\kern0pt}ASEQ\ bs\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{bsimp{\isacharunderscore}{\kern0pt}ASEQ\ bs\ {\isacharparenleft}{\kern0pt}bsimp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}bsimp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{bsimp\ {\isacharparenleft}{\kern0pt}AALTs\ bs{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}\ rs{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{bsimp{\isacharunderscore}{\kern0pt}AALTs\ bs{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}\ {\isacharparenleft}{\kern0pt}distinctBy\ {\isacharparenleft}{\kern0pt}flts\ {\isacharparenleft}{\kern0pt}map\ bsimp\ rs{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ erase\ {\isasymemptyset}{\isacharparenright}{\kern0pt}}\\
+ \isa{bsimp\ AZERO} & $\dn$ & \isa{AZERO}\\
+
+\end{tabular}
+\end{center}
+
+And the two helper functions are:
+\begin{center}
+ \begin{tabular}{lcl}
+ \isa{bsimp{\isacharunderscore}{\kern0pt}AALTs\ bs\isactrlsub {\isadigit{1}}\ {\isacharbrackleft}{\kern0pt}r{\isacharbrackright}{\kern0pt}} & $\dn$ & \isa{bsimp{\isacharunderscore}{\kern0pt}ASEQ\ bs\isactrlsub {\isadigit{1}}\ {\isacharparenleft}{\kern0pt}bsimp\ r{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}bsimp\ r{\isadigit{2}}{\isachardot}{\kern0pt}{\isadigit{0}}{\isacharparenright}{\kern0pt}}\\
+ \isa{bsimp{\isacharunderscore}{\kern0pt}AALTs\ bs{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}\ {\isacharbrackleft}{\kern0pt}r{\isacharbrackright}{\kern0pt}} & $\dn$ & \isa{bsimp{\isacharunderscore}{\kern0pt}AALTs\ bs{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}\ {\isacharparenleft}{\kern0pt}distinctBy\ {\isacharparenleft}{\kern0pt}flts\ {\isacharparenleft}{\kern0pt}map\ bsimp\ rs{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ erase\ {\isasymemptyset}{\isacharparenright}{\kern0pt}}\\
+ \isa{bsimp{\isacharunderscore}{\kern0pt}AALTs\ bs{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}\ {\isacharparenleft}{\kern0pt}v\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vb\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vc{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{AZERO}\\
+
+\end{tabular}
+\end{center}
+
+
+This might sound trivial in the case of producing a YES/NO answer,
+but once we require a lexing output to be produced (which is required
+in applications like compiler front-end, malicious attack domain extraction,
+etc.), it is not straightforward if we still extract what is needed according
+to the POSIX standard.
+
+
+
+
+
+By simplification, we mean specifically the following rules:
+
+\begin{center}
+ \begin{tabular}{lcl}
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{ASEQ\ bs\ AZERO\ r\isactrlsub {\isadigit{2}}\ {\isasymleadsto}\ AZERO}}}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{ASEQ\ bs\ r\isactrlsub {\isadigit{1}}\ AZERO\ {\isasymleadsto}\ AZERO}}}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{ASEQ\ bs\ {\isacharparenleft}{\kern0pt}AONE\ bs\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymleadsto}\ fuse\ {\isacharparenleft}{\kern0pt}bs\ {\isacharat}{\kern0pt}\ bs\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ r\isactrlsub {\isadigit{1}}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{r\isactrlsub {\isadigit{1}}\ {\isasymleadsto}\ r\isactrlsub {\isadigit{2}}}}{\mbox{ASEQ\ bs\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{3}}\ {\isasymleadsto}\ ASEQ\ bs\ r\isactrlsub {\isadigit{2}}\ r\isactrlsub {\isadigit{3}}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{r\isactrlsub {\isadigit{3}}\ {\isasymleadsto}\ r\isactrlsub {\isadigit{4}}}}{\mbox{ASEQ\ bs\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{3}}\ {\isasymleadsto}\ ASEQ\ bs\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{4}}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{r\ {\isasymleadsto}\ r{\isacharprime}{\kern0pt}}}{\mbox{AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}r{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ rs\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymleadsto}\ AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}r{\isacharprime}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ rs\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub a\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}AZERO{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ rs\isactrlsub b{\isacharparenright}{\kern0pt}\ {\isasymleadsto}\ AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub a\ {\isacharat}{\kern0pt}\ rs\isactrlsub b{\isacharparenright}{\kern0pt}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub a\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}AALTs\ bs\isactrlsub {\isadigit{1}}\ rs\isactrlsub {\isadigit{1}}{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ rs\isactrlsub b{\isacharparenright}{\kern0pt}\ {\isasymleadsto}\ AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub a\ {\isacharat}{\kern0pt}\ map\ {\isacharparenleft}{\kern0pt}fuse\ bs\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ rs\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ rs\isactrlsub b{\isacharparenright}{\kern0pt}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{AALTs\ {\isacharparenleft}{\kern0pt}bs\ {\isacharat}{\kern0pt}\ bs\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ rs\ {\isasymleadsto}\ AALTs\ bs\ {\isacharparenleft}{\kern0pt}map\ {\isacharparenleft}{\kern0pt}fuse\ bs\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ rs{\isacharparenright}{\kern0pt}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{AALTs\ bs\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymleadsto}\ AZERO}}}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{AALTs\ bs\ {\isacharbrackleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbrackright}{\kern0pt}\ {\isasymleadsto}\ fuse\ bs\ r\isactrlsub {\isadigit{1}}}}}\\
+ \isa{\mbox{}\inferrule{\mbox{a\isactrlsub {\isadigit{1}}\mbox{$^\downarrow$}\ {\isacharequal}{\kern0pt}\ a\isactrlsub {\isadigit{2}}\mbox{$^\downarrow$}}}{\mbox{AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub a\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}a\isactrlsub {\isadigit{1}}{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ rs\isactrlsub b\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}a\isactrlsub {\isadigit{2}}{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ rs\isactrlsub c{\isacharparenright}{\kern0pt}\ {\isasymleadsto}\ AALTs\ bs\ {\isacharparenleft}{\kern0pt}rs\isactrlsub a\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}a\isactrlsub {\isadigit{1}}{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ rs\isactrlsub b\ {\isacharat}{\kern0pt}\ rs\isactrlsub c{\isacharparenright}{\kern0pt}}}}\\
+
+ \end{tabular}
+\end{center}
+
+
+And these can be made compact by the following simplification function:
+
+where the function $\mathit{bsimp_AALTs}$
+
+The core idea of the proof is that two regular expressions,
+if "isomorphic" up to a finite number of rewrite steps, will
+remain "isomorphic" when we take the same sequence of
+derivatives on both of them.
+This can be expressed by the following rewrite relation lemma:
+\begin{lemma}
+\isa{{\isacharparenleft}{\kern0pt}r\mbox{$\bbslash$}s{\isacharparenright}{\kern0pt}\ {\isasymleadsto}{\isacharasterisk}{\kern0pt}\ bders{\isacharunderscore}{\kern0pt}simp\ r\ s}
+\end{lemma}
+
+This isomorphic relation implies a property that leads to the
+correctness result:
+if two (nullable) regular expressions are "rewritable" in many steps
+from one another,
+then a call to function $\textit{bmkeps}$ gives the same
+bit-sequence :
+\begin{lemma}
+\isa{{\normalsize{}If\,}\ \mbox{r{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}\ {\isasymleadsto}{\isacharasterisk}{\kern0pt}\ r{\isadigit{2}}{\isachardot}{\kern0pt}{\isadigit{0}}}\ {\normalsize \,and\,}\ \mbox{nullable\mbox{$_b$}\ r{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}}\ {\normalsize \,then\,}\ mkeps\mbox{$_b$}\ r{\isadigit{1}}{\isachardot}{\kern0pt}{\isadigit{0}}\ {\isacharequal}{\kern0pt}\ mkeps\mbox{$_b$}\ r{\isadigit{2}}{\isachardot}{\kern0pt}{\isadigit{0}}{\isachardot}{\kern0pt}}
+\end{lemma}
+
+Given the same bit-sequence, the decode function
+will give out the same value, which is the output
+of both lexers:
+\begin{lemma}
+\isa{lexer\mbox{$_b$}\ r\ s\ {\isasymequiv}\ \textrm{if}\ nullable\mbox{$_b$}\ {\isacharparenleft}{\kern0pt}r\mbox{$^\uparrow$}\mbox{$\bbslash$}s{\isacharparenright}{\kern0pt}\ \textrm{then}\ decode\ {\isacharparenleft}{\kern0pt}mkeps\mbox{$_b$}\ {\isacharparenleft}{\kern0pt}r\mbox{$^\uparrow$}\mbox{$\bbslash$}s{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ r\ \textrm{else}\ None}
+\end{lemma}
+
+\begin{lemma}
+\isa{blexer{\isacharunderscore}{\kern0pt}simp\ r\ s\ {\isasymequiv}\ \textrm{if}\ nullable\mbox{$_b$}\ {\isacharparenleft}{\kern0pt}bders{\isacharunderscore}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r\mbox{$^\uparrow$}{\isacharparenright}{\kern0pt}\ s{\isacharparenright}{\kern0pt}\ \textrm{then}\ decode\ {\isacharparenleft}{\kern0pt}mkeps\mbox{$_b$}\ {\isacharparenleft}{\kern0pt}bders{\isacharunderscore}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r\mbox{$^\uparrow$}{\isacharparenright}{\kern0pt}\ s{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ r\ \textrm{else}\ None}
+\end{lemma}
+
+And that yields the correctness result:
+\begin{lemma}
+\isa{lexer\ r\ s\ {\isacharequal}{\kern0pt}\ blexer{\isacharunderscore}{\kern0pt}simp\ r\ s}
+\end{lemma}
+
+The nice thing about the above%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{Additional Simp Rules?%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+One question someone would ask is:
+can we add more "atomic" simplification/rewriting rules,
+so the simplification is even more aggressive, making
+the intermediate results smaller, and therefore more space-efficient?
+For example, one might want to do open up alternatives who is a child
+of a sequence:
+
+\begin{center}
+ \begin{tabular}{lcl}
+ \isa{ASEQ\ bs\ {\isacharparenleft}{\kern0pt}AALTs\ bs{\isadigit{1}}\ rs{\isacharparenright}{\kern0pt}\ r\ {\isasymleadsto}{\isacharquery}{\kern0pt}\ AALTs\ {\isacharparenleft}{\kern0pt}bs\ {\isacharat}{\kern0pt}\ bs{\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}map\ {\isacharparenleft}{\kern0pt}{\isasymlambda}r{\isacharprime}{\kern0pt}{\isachardot}{\kern0pt}\ ASEQ\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ r{\isacharprime}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ rs{\isacharparenright}{\kern0pt}}\\
+ \end{tabular}
+\end{center}
+
+This rule allows us to simplify \mbox{\isa{a\ {\isacharplus}{\kern0pt}\ b\ {\isasymcdot}\ c\ {\isacharplus}{\kern0pt}\ a\ {\isasymcdot}\ c}}%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{Introduction%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+Brzozowski \cite{Brzozowski1964} introduced the notion of the {\em
+derivative} \isa{r{\isacharbackslash}{\kern0pt}c} of a regular expression \isa{r} w.r.t.\
+a character~\isa{c}, and showed that it gave a simple solution to the
+problem of matching a string \isa{s} with a regular expression \isa{r}: if the derivative of \isa{r} w.r.t.\ (in succession) all the
+characters of the string matches the empty string, then \isa{r}
+matches \isa{s} (and {\em vice versa}). The derivative has the
+property (which may almost be regarded as its specification) that, for
+every string \isa{s} and regular expression \isa{r} and character
+\isa{c}, one has \isa{cs\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}} if and only if \mbox{\isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}}}.
+The beauty of Brzozowski's derivatives is that
+they are neatly expressible in any functional language, and easily
+definable and reasoned about in theorem provers---the definitions just
+consist of inductive datatypes and simple recursive functions. A
+mechanised correctness proof of Brzozowski's matcher in for example HOL4
+has been mentioned by Owens and Slind~\cite{Owens2008}. Another one in
+Isabelle/HOL is part of the work by Krauss and Nipkow \cite{Krauss2011}.
+And another one in Coq is given by Coquand and Siles \cite{Coquand2012}.
+
+If a regular expression matches a string, then in general there is more
+than one way of how the string is matched. There are two commonly used
+disambiguation strategies to generate a unique answer: one is called
+GREEDY matching \cite{Frisch2004} and the other is POSIX
+matching~\cite{POSIX,Kuklewicz,OkuiSuzuki2010,Sulzmann2014,Vansummeren2006}.
+For example consider the string \isa{xy} and the regular expression
+\mbox{\isa{{\isacharparenleft}{\kern0pt}x\ {\isacharplus}{\kern0pt}\ y\ {\isacharplus}{\kern0pt}\ xy{\isacharparenright}{\kern0pt}\isactrlsup {\isasymstar}}}. Either the string can be
+matched in two `iterations' by the single letter-regular expressions
+\isa{x} and \isa{y}, or directly in one iteration by \isa{xy}. The
+first case corresponds to GREEDY matching, which first matches with the
+left-most symbol and only matches the next symbol in case of a mismatch
+(this is greedy in the sense of preferring instant gratification to
+delayed repletion). The second case is POSIX matching, which prefers the
+longest match.
+
+In the context of lexing, where an input string needs to be split up
+into a sequence of tokens, POSIX is the more natural disambiguation
+strategy for what programmers consider basic syntactic building blocks
+in their programs. These building blocks are often specified by some
+regular expressions, say \isa{r\isactrlbsub key\isactrlesub } and \isa{r\isactrlbsub id\isactrlesub } for recognising keywords and identifiers,
+respectively. There are a few underlying (informal) rules behind
+tokenising a string in a POSIX \cite{POSIX} fashion:
+
+\begin{itemize}
+\item[$\bullet$] \emph{The Longest Match Rule} (or \emph{``{M}aximal {M}unch {R}ule''}):
+The longest initial substring matched by any regular expression is taken as
+next token.\smallskip
+
+\item[$\bullet$] \emph{Priority Rule:}
+For a particular longest initial substring, the first (leftmost) regular expression
+that can match determines the token.\smallskip
+
+\item[$\bullet$] \emph{Star Rule:} A subexpression repeated by ${}^\star$ shall
+not match an empty string unless this is the only match for the repetition.\smallskip
+
+\item[$\bullet$] \emph{Empty String Rule:} An empty string shall be considered to
+be longer than no match at all.
+\end{itemize}
+
+\noindent Consider for example a regular expression \isa{r\isactrlbsub key\isactrlesub } for recognising keywords such as \isa{if},
+\isa{then} and so on; and \isa{r\isactrlbsub id\isactrlesub }
+recognising identifiers (say, a single character followed by
+characters or numbers). Then we can form the regular expression
+\isa{{\isacharparenleft}{\kern0pt}r\isactrlbsub key\isactrlesub \ {\isacharplus}{\kern0pt}\ r\isactrlbsub id\isactrlesub {\isacharparenright}{\kern0pt}\isactrlsup {\isasymstar}}
+and use POSIX matching to tokenise strings, say \isa{iffoo} and
+\isa{if}. For \isa{iffoo} we obtain by the Longest Match Rule
+a single identifier token, not a keyword followed by an
+identifier. For \isa{if} we obtain by the Priority Rule a keyword
+token, not an identifier token---even if \isa{r\isactrlbsub id\isactrlesub }
+matches also. By the Star Rule we know \isa{{\isacharparenleft}{\kern0pt}r\isactrlbsub key\isactrlesub \ {\isacharplus}{\kern0pt}\ r\isactrlbsub id\isactrlesub {\isacharparenright}{\kern0pt}\isactrlsup {\isasymstar}} matches \isa{iffoo},
+respectively \isa{if}, in exactly one `iteration' of the star. The
+Empty String Rule is for cases where, for example, the regular expression
+\isa{{\isacharparenleft}{\kern0pt}a\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\isactrlsup {\isasymstar}} matches against the
+string \isa{bc}. Then the longest initial matched substring is the
+empty string, which is matched by both the whole regular expression
+and the parenthesised subexpression.
+
+
+One limitation of Brzozowski's matcher is that it only generates a
+YES/NO answer for whether a string is being matched by a regular
+expression. Sulzmann and Lu~\cite{Sulzmann2014} extended this matcher
+to allow generation not just of a YES/NO answer but of an actual
+matching, called a [lexical] {\em value}. Assuming a regular
+expression matches a string, values encode the information of
+\emph{how} the string is matched by the regular expression---that is,
+which part of the string is matched by which part of the regular
+expression. For this consider again the string \isa{xy} and
+the regular expression \mbox{\isa{{\isacharparenleft}{\kern0pt}x\ {\isacharplus}{\kern0pt}\ {\isacharparenleft}{\kern0pt}y\ {\isacharplus}{\kern0pt}\ xy{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\isactrlsup {\isasymstar}}}
+(this time fully parenthesised). We can view this regular expression
+as tree and if the string \isa{xy} is matched by two Star
+`iterations', then the \isa{x} is matched by the left-most
+alternative in this tree and the \isa{y} by the right-left alternative. This
+suggests to record this matching as
+
+\begin{center}
+\isa{Stars\ {\isacharbrackleft}{\kern0pt}Left\ {\isacharparenleft}{\kern0pt}Char\ x{\isacharparenright}{\kern0pt}{\isacharcomma}{\kern0pt}\ Right\ {\isacharparenleft}{\kern0pt}Left\ {\isacharparenleft}{\kern0pt}Char\ y{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isacharbrackright}{\kern0pt}}
+\end{center}
+
+\noindent where \isa{Stars}, \isa{Left}, \isa{Right} and \isa{Char} are constructors for values. \isa{Stars} records how many
+iterations were used; \isa{Left}, respectively \isa{Right}, which
+alternative is used. This `tree view' leads naturally to the idea that
+regular expressions act as types and values as inhabiting those types
+(see, for example, \cite{HosoyaVouillonPierce2005}). The value for
+matching \isa{xy} in a single `iteration', i.e.~the POSIX value,
+would look as follows
+
+\begin{center}
+\isa{Stars\ {\isacharbrackleft}{\kern0pt}Seq\ {\isacharparenleft}{\kern0pt}Char\ x{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}Char\ y{\isacharparenright}{\kern0pt}{\isacharbrackright}{\kern0pt}}
+\end{center}
+
+\noindent where \isa{Stars} has only a single-element list for the
+single iteration and \isa{Seq} indicates that \isa{xy} is matched
+by a sequence regular expression.
+
+%, which we will in what follows
+%write more formally as \isa{x\ {\isasymcdot}\ y}.
+
+
+Sulzmann and Lu give a simple algorithm to calculate a value that
+appears to be the value associated with POSIX matching. The challenge
+then is to specify that value, in an algorithm-independent fashion,
+and to show that Sulzmann and Lu's derivative-based algorithm does
+indeed calculate a value that is correct according to the
+specification. The answer given by Sulzmann and Lu
+\cite{Sulzmann2014} is to define a relation (called an ``order
+relation'') on the set of values of \isa{r}, and to show that (once
+a string to be matched is chosen) there is a maximum element and that
+it is computed by their derivative-based algorithm. This proof idea is
+inspired by work of Frisch and Cardelli \cite{Frisch2004} on a GREEDY
+regular expression matching algorithm. However, we were not able to
+establish transitivity and totality for the ``order relation'' by
+Sulzmann and Lu. There are some inherent problems with their approach
+(of which some of the proofs are not published in
+\cite{Sulzmann2014}); perhaps more importantly, we give in this paper
+a simple inductive (and algorithm-independent) definition of what we
+call being a {\em POSIX value} for a regular expression \isa{r} and
+a string \isa{s}; we show that the algorithm by Sulzmann and Lu
+computes such a value and that such a value is unique. Our proofs are
+both done by hand and checked in Isabelle/HOL. The experience of
+doing our proofs has been that this mechanical checking was absolutely
+essential: this subject area has hidden snares. This was also noted by
+Kuklewicz \cite{Kuklewicz} who found that nearly all POSIX matching
+implementations are ``buggy'' \cite[Page 203]{Sulzmann2014} and by
+Grathwohl et al \cite[Page 36]{CrashCourse2014} who wrote:
+
+\begin{quote}
+\it{}``The POSIX strategy is more complicated than the greedy because of
+the dependence on information about the length of matched strings in the
+various subexpressions.''
+\end{quote}
+
+
+
+\noindent {\bf Contributions:} We have implemented in Isabelle/HOL the
+derivative-based regular expression matching algorithm of
+Sulzmann and Lu \cite{Sulzmann2014}. We have proved the correctness of this
+algorithm according to our specification of what a POSIX value is (inspired
+by work of Vansummeren \cite{Vansummeren2006}). Sulzmann
+and Lu sketch in \cite{Sulzmann2014} an informal correctness proof: but to
+us it contains unfillable gaps.\footnote{An extended version of
+\cite{Sulzmann2014} is available at the website of its first author; this
+extended version already includes remarks in the appendix that their
+informal proof contains gaps, and possible fixes are not fully worked out.}
+Our specification of a POSIX value consists of a simple inductive definition
+that given a string and a regular expression uniquely determines this value.
+We also show that our definition is equivalent to an ordering
+of values based on positions by Okui and Suzuki \cite{OkuiSuzuki2010}.
+
+%Derivatives as calculated by Brzozowski's method are usually more complex
+%regular expressions than the initial one; various optimisations are
+%possible. We prove the correctness when simplifications of \isa{\isactrlbold {\isadigit{0}}\ {\isacharplus}{\kern0pt}\ r},
+%\isa{r\ {\isacharplus}{\kern0pt}\ \isactrlbold {\isadigit{0}}}, \isa{\isactrlbold {\isadigit{1}}\ {\isasymcdot}\ r} and \isa{r\ {\isasymcdot}\ \isactrlbold {\isadigit{1}}} to
+%\isa{r} are applied.
+
+We extend our results to ??? Bitcoded version??%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{Preliminaries%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+\noindent Strings in Isabelle/HOL are lists of characters with
+the empty string being represented by the empty list, written \isa{{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}, and list-cons being written as \isa{\underline{\hspace{2mm}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}\underline{\hspace{2mm}}}. Often
+we use the usual bracket notation for lists also for strings; for
+example a string consisting of just a single character \isa{c} is
+written \isa{{\isacharbrackleft}{\kern0pt}c{\isacharbrackright}{\kern0pt}}. We use the usual definitions for
+\emph{prefixes} and \emph{strict prefixes} of strings. By using the
+type \isa{char} for characters we have a supply of finitely many
+characters roughly corresponding to the ASCII character set. Regular
+expressions are defined as usual as the elements of the following
+inductive datatype:
+
+ \begin{center}
+ \isa{r\ {\isacharcolon}{\kern0pt}{\isacharequal}{\kern0pt}}
+ \isa{\isactrlbold {\isadigit{0}}} $\mid$
+ \isa{\isactrlbold {\isadigit{1}}} $\mid$
+ \isa{c} $\mid$
+ \isa{r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}} $\mid$
+ \isa{r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}} $\mid$
+ \isa{r\isactrlsup {\isasymstar}}
+ \end{center}
+
+ \noindent where \isa{\isactrlbold {\isadigit{0}}} stands for the regular expression that does
+ not match any string, \isa{\isactrlbold {\isadigit{1}}} for the regular expression that matches
+ only the empty string and \isa{c} for matching a character literal. The
+ language of a regular expression is also defined as usual by the
+ recursive function \isa{L} with the six clauses:
+
+ \begin{center}
+ \begin{tabular}{l@ {\hspace{4mm}}rcl}
+ \textit{(1)} & \isa{L{\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{0}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isasymemptyset}}\\
+ \textit{(2)} & \isa{L{\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{1}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}}\\
+ \textit{(3)} & \isa{L{\isacharparenleft}{\kern0pt}c{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}c{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}}\\
+ \textit{(4)} & \isa{L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ &
+ \isa{L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharat}{\kern0pt}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \textit{(5)} & \isa{L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ &
+ \isa{L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymunion}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \textit{(6)} & \isa{L{\isacharparenleft}{\kern0pt}r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isasymstar}}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent In clause \textit{(4)} we use the operation \isa{\underline{\hspace{2mm}}\ {\isacharat}{\kern0pt}\ \underline{\hspace{2mm}}} for the concatenation of two languages (it is also list-append for
+ strings). We use the star-notation for regular expressions and for
+ languages (in the last clause above). The star for languages is defined
+ inductively by two clauses: \isa{{\isacharparenleft}{\kern0pt}i{\isacharparenright}{\kern0pt}} the empty string being in
+ the star of a language and \isa{{\isacharparenleft}{\kern0pt}ii{\isacharparenright}{\kern0pt}} if \isa{s\isactrlsub {\isadigit{1}}} is in a
+ language and \isa{s\isactrlsub {\isadigit{2}}} in the star of this language, then also \isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}} is in the star of this language. It will also be convenient
+ to use the following notion of a \emph{semantic derivative} (or \emph{left
+ quotient}) of a language defined as
+ %
+ \begin{center}
+ \isa{Der\ c\ A\ {\isasymequiv}\ {\isacharbraceleft}{\kern0pt}s\ \mbox{\boldmath$\mid$}\ c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s\ {\isasymin}\ A{\isacharbraceright}{\kern0pt}}\;.
+ \end{center}
+
+ \noindent
+ For semantic derivatives we have the following equations (for example
+ mechanically proved in \cite{Krauss2011}):
+ %
+ \begin{equation}\label{SemDer}
+ \begin{array}{lcl}
+ \isa{Der\ c\ {\isasymemptyset}} & \dn & \isa{{\isasymemptyset}}\\
+ \isa{Der\ c\ {\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}} & \dn & \isa{{\isasymemptyset}}\\
+ \isa{Der\ c\ {\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}d{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}} & \dn & \isa{\textrm{if}\ c\ {\isacharequal}{\kern0pt}\ d\ \textrm{then}\ {\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ \textrm{else}\ {\isasymemptyset}}\\
+ \isa{Der\ c\ {\isacharparenleft}{\kern0pt}A\ {\isasymunion}\ B{\isacharparenright}{\kern0pt}} & \dn & \isa{Der\ c\ A\ {\isasymunion}\ Der\ c\ B}\\
+ \isa{Der\ c\ {\isacharparenleft}{\kern0pt}A\ {\isacharat}{\kern0pt}\ B{\isacharparenright}{\kern0pt}} & \dn & \isa{{\isacharparenleft}{\kern0pt}Der\ c\ A\ {\isacharat}{\kern0pt}\ B{\isacharparenright}{\kern0pt}\ {\isasymunion}\ {\isacharparenleft}{\kern0pt}\textrm{if}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymin}\ A\ \textrm{then}\ Der\ c\ B\ \textrm{else}\ {\isasymemptyset}{\isacharparenright}{\kern0pt}}\\
+ \isa{Der\ c\ {\isacharparenleft}{\kern0pt}A{\isasymstar}{\isacharparenright}{\kern0pt}} & \dn & \isa{Der\ c\ A\ {\isacharat}{\kern0pt}\ A{\isasymstar}}
+ \end{array}
+ \end{equation}
+
+
+ \noindent \emph{\Brz's derivatives} of regular expressions
+ \cite{Brzozowski1964} can be easily defined by two recursive functions:
+ the first is from regular expressions to booleans (implementing a test
+ when a regular expression can match the empty string), and the second
+ takes a regular expression and a character to a (derivative) regular
+ expression:
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{nullable\ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{0}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{False}\\
+ \isa{nullable\ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{1}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{True}\\
+ \isa{nullable\ {\isacharparenleft}{\kern0pt}c{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{False}\\
+ \isa{nullable\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{nullable\ r\isactrlsub {\isadigit{1}}\ {\isasymor}\ nullable\ r\isactrlsub {\isadigit{2}}}\\
+ \isa{nullable\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{nullable\ r\isactrlsub {\isadigit{1}}\ {\isasymand}\ nullable\ r\isactrlsub {\isadigit{2}}}\\
+ \isa{nullable\ {\isacharparenleft}{\kern0pt}r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{True}\medskip\\
+
+% \end{tabular}
+% \end{center}
+
+% \begin{center}
+% \begin{tabular}{lcl}
+
+ \isa{\isactrlbold {\isadigit{0}}{\isacharbackslash}{\kern0pt}c} & $\dn$ & \isa{\isactrlbold {\isadigit{0}}}\\
+ \isa{\isactrlbold {\isadigit{1}}{\isacharbackslash}{\kern0pt}c} & $\dn$ & \isa{\isactrlbold {\isadigit{0}}}\\
+ \isa{d{\isacharbackslash}{\kern0pt}c} & $\dn$ & \isa{\textrm{if}\ c\ {\isacharequal}{\kern0pt}\ d\ \textrm{then}\ \isactrlbold {\isadigit{1}}\ \textrm{else}\ \isactrlbold {\isadigit{0}}}\\
+ \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isacharplus}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}}\\
+ \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c} & $\dn$ & \isa{\textrm{if}\ nullable\ r\isactrlsub {\isadigit{1}}\ \textrm{then}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}\ {\isacharplus}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ \textrm{else}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}}\\
+ \isa{{\isacharparenleft}{\kern0pt}r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsup {\isasymstar}}
+ \end{tabular}
+ \end{center}
+
+ \noindent
+ We may extend this definition to give derivatives w.r.t.~strings:
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{r{\isacharbackslash}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} & $\dn$ & \isa{r}\\
+ \isa{r{\isacharbackslash}{\kern0pt}{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}s}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent Given the equations in \eqref{SemDer}, it is a relatively easy
+ exercise in mechanical reasoning to establish that
+
+ \begin{proposition}\label{derprop}\mbox{}\\
+ \begin{tabular}{ll}
+ \textit{(1)} & \isa{nullable\ r} if and only if
+ \isa{{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}}, and \\
+ \textit{(2)} & \isa{L{\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ Der\ c\ {\isacharparenleft}{\kern0pt}L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}}.
+ \end{tabular}
+ \end{proposition}
+
+ \noindent With this in place it is also very routine to prove that the
+ regular expression matcher defined as
+ %
+ \begin{center}
+ \isa{match\ r\ s\ {\isasymequiv}\ nullable\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}s{\isacharparenright}{\kern0pt}}
+ \end{center}
+
+ \noindent gives a positive answer if and only if \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}}.
+ Consequently, this regular expression matching algorithm satisfies the
+ usual specification for regular expression matching. While the matcher
+ above calculates a provably correct YES/NO answer for whether a regular
+ expression matches a string or not, the novel idea of Sulzmann and Lu
+ \cite{Sulzmann2014} is to append another phase to this algorithm in order
+ to calculate a [lexical] value. We will explain the details next.%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{POSIX Regular Expression Matching\label{posixsec}%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+There have been many previous works that use values for encoding
+ \emph{how} a regular expression matches a string.
+ The clever idea by Sulzmann and Lu \cite{Sulzmann2014} is to
+ define a function on values that mirrors (but inverts) the
+ construction of the derivative on regular expressions. \emph{Values}
+ are defined as the inductive datatype
+
+ \begin{center}
+ \isa{v\ {\isacharcolon}{\kern0pt}{\isacharequal}{\kern0pt}}
+ \isa{Empty} $\mid$
+ \isa{Char\ c} $\mid$
+ \isa{Left\ v} $\mid$
+ \isa{Right\ v} $\mid$
+ \isa{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}} $\mid$
+ \isa{Stars\ vs}
+ \end{center}
+
+ \noindent where we use \isa{vs} to stand for a list of
+ values. (This is similar to the approach taken by Frisch and
+ Cardelli for GREEDY matching \cite{Frisch2004}, and Sulzmann and Lu
+ for POSIX matching \cite{Sulzmann2014}). The string underlying a
+ value can be calculated by the \isa{flat} function, written
+ \isa{{\isacharbar}{\kern0pt}\underline{\hspace{2mm}}{\isacharbar}{\kern0pt}} and defined as:
+
+ \begin{center}
+ \begin{tabular}[t]{lcl}
+ \isa{{\isacharbar}{\kern0pt}Empty{\isacharbar}{\kern0pt}} & $\dn$ & \isa{{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}\\
+ \isa{{\isacharbar}{\kern0pt}Char\ c{\isacharbar}{\kern0pt}} & $\dn$ & \isa{{\isacharbrackleft}{\kern0pt}c{\isacharbrackright}{\kern0pt}}\\
+ \isa{{\isacharbar}{\kern0pt}Left\ v{\isacharbar}{\kern0pt}} & $\dn$ & \isa{{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}}\\
+ \isa{{\isacharbar}{\kern0pt}Right\ v{\isacharbar}{\kern0pt}} & $\dn$ & \isa{{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}}
+ \end{tabular}\hspace{14mm}
+ \begin{tabular}[t]{lcl}
+ \isa{{\isacharbar}{\kern0pt}Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} & $\dn$ & \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharat}{\kern0pt}\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}}\\
+ \isa{{\isacharbar}{\kern0pt}Stars\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbar}{\kern0pt}} & $\dn$ & \isa{{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}\\
+ \isa{{\isacharbar}{\kern0pt}Stars\ {\isacharparenleft}{\kern0pt}v\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs{\isacharparenright}{\kern0pt}{\isacharbar}{\kern0pt}} & $\dn$ & \isa{{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}\ {\isacharat}{\kern0pt}\ {\isacharbar}{\kern0pt}Stars\ vs{\isacharbar}{\kern0pt}}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent We will sometimes refer to the underlying string of a
+ value as \emph{flattened value}. We will also overload our notation and
+ use \isa{{\isacharbar}{\kern0pt}vs{\isacharbar}{\kern0pt}} for flattening a list of values and concatenating
+ the resulting strings.
+
+ Sulzmann and Lu define
+ inductively an \emph{inhabitation relation} that associates values to
+ regular expressions. We define this relation as
+ follows:\footnote{Note that the rule for \isa{Stars} differs from
+ our earlier paper \cite{AusafDyckhoffUrban2016}. There we used the
+ original definition by Sulzmann and Lu which does not require that
+ the values \isa{v\ {\isasymin}\ vs} flatten to a non-empty
+ string. The reason for introducing the more restricted version of
+ lexical values is convenience later on when reasoning about an
+ ordering relation for values.}
+
+ \begin{center}
+ \begin{tabular}{c@ {\hspace{12mm}}c}\label{prfintros}
+ \\[-8mm]
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{Empty\ {\isacharcolon}{\kern0pt}\ \isactrlbold {\isadigit{1}}}}} &
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{Char\ c\ {\isacharcolon}{\kern0pt}\ c}}}\\[4mm]
+ \isa{\mbox{}\inferrule{\mbox{v\isactrlsub {\isadigit{1}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{1}}}}{\mbox{Left\ v\isactrlsub {\isadigit{1}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}}}} &
+ \isa{\mbox{}\inferrule{\mbox{v\isactrlsub {\isadigit{2}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{1}}}}{\mbox{Right\ v\isactrlsub {\isadigit{2}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{2}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{1}}}}}\\[4mm]
+ \isa{\mbox{}\inferrule{\mbox{v\isactrlsub {\isadigit{1}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{1}}}\\\ \mbox{v\isactrlsub {\isadigit{2}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{2}}}}{\mbox{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}}}} &
+ \isa{\mbox{}\inferrule{\mbox{{\isasymforall}v{\isasymin}vs{\isachardot}{\kern0pt}\ v\ {\isacharcolon}{\kern0pt}\ r\ {\isasymand}\ {\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}}{\mbox{Stars\ vs\ {\isacharcolon}{\kern0pt}\ r\isactrlsup {\isasymstar}}}}
+ \end{tabular}
+ \end{center}
+
+ \noindent where in the clause for \isa{Stars} we use the
+ notation \isa{v\ {\isasymin}\ vs} for indicating that \isa{v} is a
+ member in the list \isa{vs}. We require in this rule that every
+ value in \isa{vs} flattens to a non-empty string. The idea is that
+ \isa{Stars}-values satisfy the informal Star Rule (see Introduction)
+ where the $^\star$ does not match the empty string unless this is
+ the only match for the repetition. Note also that no values are
+ associated with the regular expression \isa{\isactrlbold {\isadigit{0}}}, and that the
+ only value associated with the regular expression \isa{\isactrlbold {\isadigit{1}}} is
+ \isa{Empty}. It is routine to establish how values ``inhabiting''
+ a regular expression correspond to the language of a regular
+ expression, namely
+
+ \begin{proposition}\label{inhabs}
+ \isa{L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbraceleft}{\kern0pt}{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}\ \mbox{\boldmath$\mid$}\ v\ {\isacharcolon}{\kern0pt}\ r{\isacharbraceright}{\kern0pt}}
+ \end{proposition}
+
+ \noindent
+ Given a regular expression \isa{r} and a string \isa{s}, we define the
+ set of all \emph{Lexical Values} inhabited by \isa{r} with the underlying string
+ being \isa{s}:\footnote{Okui and Suzuki refer to our lexical values
+ as \emph{canonical values} in \cite{OkuiSuzuki2010}. The notion of \emph{non-problematic
+ values} by Cardelli and Frisch \cite{Frisch2004} is related, but not identical
+ to our lexical values.}
+
+ \begin{center}
+ \isa{LV\ r\ s\ {\isasymequiv}\ {\isacharbraceleft}{\kern0pt}v\ \mbox{\boldmath$\mid$}\ v\ {\isacharcolon}{\kern0pt}\ r\ {\isasymand}\ {\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ s{\isacharbraceright}{\kern0pt}}
+ \end{center}
+
+ \noindent The main property of \isa{LV\ r\ s} is that it is alway finite.
+
+ \begin{proposition}
+ \isa{finite\ {\isacharparenleft}{\kern0pt}LV\ r\ s{\isacharparenright}{\kern0pt}}
+ \end{proposition}
+
+ \noindent This finiteness property does not hold in general if we
+ remove the side-condition about \isa{{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} in the
+ \isa{Stars}-rule above. For example using Sulzmann and Lu's
+ less restrictive definition, \isa{LV\ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{1}}\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} would contain
+ infinitely many values, but according to our more restricted
+ definition only a single value, namely \isa{LV\ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{1}}\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbraceleft}{\kern0pt}Stars\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}}.
+
+ If a regular expression \isa{r} matches a string \isa{s}, then
+ generally the set \isa{LV\ r\ s} is not just a singleton set. In
+ case of POSIX matching the problem is to calculate the unique lexical value
+ that satisfies the (informal) POSIX rules from the Introduction.
+ Graphically the POSIX value calculation algorithm by Sulzmann and Lu
+ can be illustrated by the picture in Figure~\ref{Sulz} where the
+ path from the left to the right involving \isa{derivatives}/\isa{nullable} is the first phase of the algorithm
+ (calculating successive \Brz's derivatives) and \isa{mkeps}/\isa{inj}, the path from right to left, the second
+ phase. This picture shows the steps required when a regular
+ expression, say \isa{r\isactrlsub {\isadigit{1}}}, matches the string \isa{{\isacharbrackleft}{\kern0pt}a{\isacharcomma}{\kern0pt}\ b{\isacharcomma}{\kern0pt}\ c{\isacharbrackright}{\kern0pt}}. We first build the three derivatives (according to
+ \isa{a}, \isa{b} and \isa{c}). We then use \isa{nullable}
+ to find out whether the resulting derivative regular expression
+ \isa{r\isactrlsub {\isadigit{4}}} can match the empty string. If yes, we call the
+ function \isa{mkeps} that produces a value \isa{v\isactrlsub {\isadigit{4}}}
+ for how \isa{r\isactrlsub {\isadigit{4}}} can match the empty string (taking into
+ account the POSIX constraints in case there are several ways). This
+ function is defined by the clauses:
+
+\begin{figure}[t]
+\begin{center}
+\begin{tikzpicture}[scale=2,node distance=1.3cm,
+ every node/.style={minimum size=6mm}]
+\node (r1) {\isa{r\isactrlsub {\isadigit{1}}}};
+\node (r2) [right=of r1]{\isa{r\isactrlsub {\isadigit{2}}}};
+\draw[->,line width=1mm](r1)--(r2) node[above,midway] {\isa{\underline{\hspace{2mm}}{\isacharbackslash}{\kern0pt}a}};
+\node (r3) [right=of r2]{\isa{r\isactrlsub {\isadigit{3}}}};
+\draw[->,line width=1mm](r2)--(r3) node[above,midway] {\isa{\underline{\hspace{2mm}}{\isacharbackslash}{\kern0pt}b}};
+\node (r4) [right=of r3]{\isa{r\isactrlsub {\isadigit{4}}}};
+\draw[->,line width=1mm](r3)--(r4) node[above,midway] {\isa{\underline{\hspace{2mm}}{\isacharbackslash}{\kern0pt}c}};
+\draw (r4) node[anchor=west] {\;\raisebox{3mm}{\isa{nullable}}};
+\node (v4) [below=of r4]{\isa{v\isactrlsub {\isadigit{4}}}};
+\draw[->,line width=1mm](r4) -- (v4);
+\node (v3) [left=of v4] {\isa{v\isactrlsub {\isadigit{3}}}};
+\draw[->,line width=1mm](v4)--(v3) node[below,midway] {\isa{inj\ r\isactrlsub {\isadigit{3}}\ c}};
+\node (v2) [left=of v3]{\isa{v\isactrlsub {\isadigit{2}}}};
+\draw[->,line width=1mm](v3)--(v2) node[below,midway] {\isa{inj\ r\isactrlsub {\isadigit{2}}\ b}};
+\node (v1) [left=of v2] {\isa{v\isactrlsub {\isadigit{1}}}};
+\draw[->,line width=1mm](v2)--(v1) node[below,midway] {\isa{inj\ r\isactrlsub {\isadigit{1}}\ a}};
+\draw (r4) node[anchor=north west] {\;\raisebox{-8mm}{\isa{mkeps}}};
+\end{tikzpicture}
+\end{center}
+\mbox{}\\[-13mm]
+
+\caption{The two phases of the algorithm by Sulzmann \& Lu \cite{Sulzmann2014},
+matching the string \isa{{\isacharbrackleft}{\kern0pt}a{\isacharcomma}{\kern0pt}\ b{\isacharcomma}{\kern0pt}\ c{\isacharbrackright}{\kern0pt}}. The first phase (the arrows from
+left to right) is \Brz's matcher building successive derivatives. If the
+last regular expression is \isa{nullable}, then the functions of the
+second phase are called (the top-down and right-to-left arrows): first
+\isa{mkeps} calculates a value \isa{v\isactrlsub {\isadigit{4}}} witnessing
+how the empty string has been recognised by \isa{r\isactrlsub {\isadigit{4}}}. After
+that the function \isa{inj} ``injects back'' the characters of the string into
+the values.
+\label{Sulz}}
+\end{figure}
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{mkeps\ \isactrlbold {\isadigit{1}}} & $\dn$ & \isa{Empty}\\
+ \isa{mkeps\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{Seq\ {\isacharparenleft}{\kern0pt}mkeps\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}mkeps\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{mkeps\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{\textrm{if}\ nullable\ r\isactrlsub {\isadigit{1}}\ \textrm{then}\ Left\ {\isacharparenleft}{\kern0pt}mkeps\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ \textrm{else}\ Right\ {\isacharparenleft}{\kern0pt}mkeps\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{mkeps\ {\isacharparenleft}{\kern0pt}r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{Stars\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent Note that this function needs only to be partially defined,
+ namely only for regular expressions that are nullable. In case \isa{nullable} fails, the string \isa{{\isacharbrackleft}{\kern0pt}a{\isacharcomma}{\kern0pt}\ b{\isacharcomma}{\kern0pt}\ c{\isacharbrackright}{\kern0pt}} cannot be matched by \isa{r\isactrlsub {\isadigit{1}}} and the null value \isa{None} is returned. Note also how this function
+ makes some subtle choices leading to a POSIX value: for example if an
+ alternative regular expression, say \isa{r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}}, can
+ match the empty string and furthermore \isa{r\isactrlsub {\isadigit{1}}} can match the
+ empty string, then we return a \isa{Left}-value. The \isa{Right}-value will only be returned if \isa{r\isactrlsub {\isadigit{1}}} cannot match the empty
+ string.
+
+ The most interesting idea from Sulzmann and Lu \cite{Sulzmann2014} is
+ the construction of a value for how \isa{r\isactrlsub {\isadigit{1}}} can match the
+ string \isa{{\isacharbrackleft}{\kern0pt}a{\isacharcomma}{\kern0pt}\ b{\isacharcomma}{\kern0pt}\ c{\isacharbrackright}{\kern0pt}} from the value how the last derivative, \isa{r\isactrlsub {\isadigit{4}}} in Fig.~\ref{Sulz}, can match the empty string. Sulzmann and
+ Lu achieve this by stepwise ``injecting back'' the characters into the
+ values thus inverting the operation of building derivatives, but on the level
+ of values. The corresponding function, called \isa{inj}, takes three
+ arguments, a regular expression, a character and a value. For example in
+ the first (or right-most) \isa{inj}-step in Fig.~\ref{Sulz} the regular
+ expression \isa{r\isactrlsub {\isadigit{3}}}, the character \isa{c} from the last
+ derivative step and \isa{v\isactrlsub {\isadigit{4}}}, which is the value corresponding
+ to the derivative regular expression \isa{r\isactrlsub {\isadigit{4}}}. The result is
+ the new value \isa{v\isactrlsub {\isadigit{3}}}. The final result of the algorithm is
+ the value \isa{v\isactrlsub {\isadigit{1}}}. The \isa{inj} function is defined by recursion on regular
+ expressions and by analysing the shape of values (corresponding to
+ the derivative regular expressions).
+ %
+ \begin{center}
+ \begin{tabular}{l@ {\hspace{5mm}}lcl}
+ \textit{(1)} & \isa{inj\ d\ c\ {\isacharparenleft}{\kern0pt}Empty{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{Char\ d}\\
+ \textit{(2)} & \isa{inj\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ c\ {\isacharparenleft}{\kern0pt}Left\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}} & $\dn$ &
+ \isa{Left\ {\isacharparenleft}{\kern0pt}inj\ r\isactrlsub {\isadigit{1}}\ c\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}}\\
+ \textit{(3)} & \isa{inj\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ c\ {\isacharparenleft}{\kern0pt}Right\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ &
+ \isa{Right\ {\isacharparenleft}{\kern0pt}inj\ r\isactrlsub {\isadigit{2}}\ c\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \textit{(4)} & \isa{inj\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ c\ {\isacharparenleft}{\kern0pt}Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$
+ & \isa{Seq\ {\isacharparenleft}{\kern0pt}inj\ r\isactrlsub {\isadigit{1}}\ c\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ v\isactrlsub {\isadigit{2}}}\\
+ \textit{(5)} & \isa{inj\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ c\ {\isacharparenleft}{\kern0pt}Left\ {\isacharparenleft}{\kern0pt}Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}} & $\dn$
+ & \isa{Seq\ {\isacharparenleft}{\kern0pt}inj\ r\isactrlsub {\isadigit{1}}\ c\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ v\isactrlsub {\isadigit{2}}}\\
+ \textit{(6)} & \isa{inj\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ c\ {\isacharparenleft}{\kern0pt}Right\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$
+ & \isa{Seq\ {\isacharparenleft}{\kern0pt}mkeps\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}inj\ r\isactrlsub {\isadigit{2}}\ c\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \textit{(7)} & \isa{inj\ {\isacharparenleft}{\kern0pt}r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\ c\ {\isacharparenleft}{\kern0pt}Seq\ v\ {\isacharparenleft}{\kern0pt}Stars\ vs{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}} & $\dn$
+ & \isa{Stars\ {\isacharparenleft}{\kern0pt}inj\ r\ c\ v\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs{\isacharparenright}{\kern0pt}}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent To better understand what is going on in this definition it
+ might be instructive to look first at the three sequence cases (clauses
+ \textit{(4)} -- \textit{(6)}). In each case we need to construct an ``injected value'' for
+ \isa{r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}}. This must be a value of the form \isa{Seq\ \underline{\hspace{2mm}}\ \underline{\hspace{2mm}}}\,. Recall the clause of the \isa{derivative}-function
+ for sequence regular expressions:
+
+ \begin{center}
+ \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c} $\dn$ \isa{\textrm{if}\ nullable\ r\isactrlsub {\isadigit{1}}\ \textrm{then}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}\ {\isacharplus}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ \textrm{else}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}}
+ \end{center}
+
+ \noindent Consider first the \isa{else}-branch where the derivative is \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}}. The corresponding value must therefore
+ be of the form \isa{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}}, which matches the left-hand
+ side in clause~\textit{(4)} of \isa{inj}. In the \isa{if}-branch the derivative is an
+ alternative, namely \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}\ {\isacharplus}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}}. This means we either have to consider a \isa{Left}- or
+ \isa{Right}-value. In case of the \isa{Left}-value we know further it
+ must be a value for a sequence regular expression. Therefore the pattern
+ we match in the clause \textit{(5)} is \isa{Left\ {\isacharparenleft}{\kern0pt}Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}},
+ while in \textit{(6)} it is just \isa{Right\ v\isactrlsub {\isadigit{2}}}. One more interesting
+ point is in the right-hand side of clause \textit{(6)}: since in this case the
+ regular expression \isa{r\isactrlsub {\isadigit{1}}} does not ``contribute'' to
+ matching the string, that means it only matches the empty string, we need to
+ call \isa{mkeps} in order to construct a value for how \isa{r\isactrlsub {\isadigit{1}}}
+ can match this empty string. A similar argument applies for why we can
+ expect in the left-hand side of clause \textit{(7)} that the value is of the form
+ \isa{Seq\ v\ {\isacharparenleft}{\kern0pt}Stars\ vs{\isacharparenright}{\kern0pt}}---the derivative of a star is \isa{{\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsup {\isasymstar}}. Finally, the reason for why we can ignore the second argument
+ in clause \textit{(1)} of \isa{inj} is that it will only ever be called in cases
+ where \isa{c\ {\isacharequal}{\kern0pt}\ d}, but the usual linearity restrictions in patterns do
+ not allow us to build this constraint explicitly into our function
+ definition.\footnote{Sulzmann and Lu state this clause as \isa{inj\ c\ c\ {\isacharparenleft}{\kern0pt}Empty{\isacharparenright}{\kern0pt}} $\dn$ \isa{Char\ c},
+ but our deviation is harmless.}
+
+ The idea of the \isa{inj}-function to ``inject'' a character, say
+ \isa{c}, into a value can be made precise by the first part of the
+ following lemma, which shows that the underlying string of an injected
+ value has a prepended character \isa{c}; the second part shows that
+ the underlying string of an \isa{mkeps}-value is always the empty
+ string (given the regular expression is nullable since otherwise
+ \isa{mkeps} might not be defined).
+
+ \begin{lemma}\mbox{}\smallskip\\\label{Prf_injval_flat}
+ \begin{tabular}{ll}
+ (1) & \isa{{\normalsize{}If\,}\ v\ {\isacharcolon}{\kern0pt}\ r{\isacharbackslash}{\kern0pt}c\ {\normalsize \,then\,}\ {\isacharbar}{\kern0pt}inj\ r\ c\ v{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}{\isachardot}{\kern0pt}}\\
+ (2) & \isa{{\normalsize{}If\,}\ nullable\ r\ {\normalsize \,then\,}\ {\isacharbar}{\kern0pt}mkeps\ r{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isachardot}{\kern0pt}}
+ \end{tabular}
+ \end{lemma}
+
+ \begin{proof}
+ Both properties are by routine inductions: the first one can, for example,
+ be proved by induction over the definition of \isa{derivatives}; the second by
+ an induction on \isa{r}. There are no interesting cases.\qed
+ \end{proof}
+
+ Having defined the \isa{mkeps} and \isa{inj} function we can extend
+ \Brz's matcher so that a value is constructed (assuming the
+ regular expression matches the string). The clauses of the Sulzmann and Lu lexer are
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{lexer\ r\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} & $\dn$ & \isa{\textrm{if}\ nullable\ r\ \textrm{then}\ Some\ {\isacharparenleft}{\kern0pt}mkeps\ r{\isacharparenright}{\kern0pt}\ \textrm{else}\ None}\\
+ \isa{lexer\ r\ {\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{case} \isa{lexer\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ s} \isa{of}\\
+ & & \phantom{$|$} \isa{None} \isa{{\isasymRightarrow}} \isa{None}\\
+ & & $|$ \isa{Some\ v} \isa{{\isasymRightarrow}} \isa{Some\ {\isacharparenleft}{\kern0pt}inj\ r\ c\ v{\isacharparenright}{\kern0pt}}
+ \end{tabular}
+ \end{center}
+
+ \noindent If the regular expression does not match the string, \isa{None} is
+ returned. If the regular expression \emph{does}
+ match the string, then \isa{Some} value is returned. One important
+ virtue of this algorithm is that it can be implemented with ease in any
+ functional programming language and also in Isabelle/HOL. In the remaining
+ part of this section we prove that this algorithm is correct.
+
+ The well-known idea of POSIX matching is informally defined by some
+ rules such as the Longest Match and Priority Rules (see
+ Introduction); as correctly argued in \cite{Sulzmann2014}, this
+ needs formal specification. Sulzmann and Lu define an ``ordering
+ relation'' between values and argue that there is a maximum value,
+ as given by the derivative-based algorithm. In contrast, we shall
+ introduce a simple inductive definition that specifies directly what
+ a \emph{POSIX value} is, incorporating the POSIX-specific choices
+ into the side-conditions of our rules. Our definition is inspired by
+ the matching relation given by Vansummeren~\cite{Vansummeren2006}.
+ The relation we define is ternary and
+ written as \mbox{\isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}}, relating
+ strings, regular expressions and values; the inductive rules are given in
+ Figure~\ref{POSIXrules}.
+ We can prove that given a string \isa{s} and regular expression \isa{r}, the POSIX value \isa{v} is uniquely determined by \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}.
+
+ %
+ \begin{figure}[t]
+ \begin{center}
+ \begin{tabular}{c}
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{{\isacharparenleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharcomma}{\kern0pt}\ \isactrlbold {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Empty}}}\isa{P}\isa{\isactrlbold {\isadigit{1}}} \qquad
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{{\isacharparenleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}c{\isacharbrackright}{\kern0pt}{\isacharcomma}{\kern0pt}\ c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Char\ c}}}\isa{P}\isa{c}\medskip\\
+ \isa{\mbox{}\inferrule{\mbox{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}}{\mbox{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Left\ v}}}\isa{P{\isacharplus}{\kern0pt}L}\qquad
+ \isa{\mbox{}\inferrule{\mbox{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}\\\ \mbox{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}}}{\mbox{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Right\ v}}}\isa{P{\isacharplus}{\kern0pt}R}\medskip\\
+ $\mprset{flushleft}
+ \inferrule
+ {\isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{1}}} \qquad
+ \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{2}}} \\\\
+ \isa{{\isasymnexists}s\isactrlsub {\isadigit{3}}\ s\isactrlsub {\isadigit{4}}{\isachardot}{\kern0pt}a{\isachardot}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{3}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{4}}\ {\isacharequal}{\kern0pt}\ s\isactrlsub {\isadigit{2}}\ {\isasymand}\ s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{4}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}}
+ {\isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}}}$\isa{PS}\\
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{{\isacharparenleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharcomma}{\kern0pt}\ r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Stars\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}}}\isa{P{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}\medskip\\
+ $\mprset{flushleft}
+ \inferrule
+ {\isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v} \qquad
+ \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Stars\ vs} \qquad
+ \isa{{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} \\\\
+ \isa{{\isasymnexists}s\isactrlsub {\isadigit{3}}\ s\isactrlsub {\isadigit{4}}{\isachardot}{\kern0pt}a{\isachardot}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{3}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{4}}\ {\isacharequal}{\kern0pt}\ s\isactrlsub {\isadigit{2}}\ {\isasymand}\ s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{4}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}}}
+ {\isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Stars\ {\isacharparenleft}{\kern0pt}v\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs{\isacharparenright}{\kern0pt}}}$\isa{P{\isasymstar}}
+ \end{tabular}
+ \end{center}
+ \caption{Our inductive definition of POSIX values.}\label{POSIXrules}
+ \end{figure}
+
+
+
+ \begin{theorem}\mbox{}\smallskip\\\label{posixdeterm}
+ \begin{tabular}{ll}
+ (1) & If \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v} then \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}} and \isa{{\isacharbar}{\kern0pt}v{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ s}.\\
+ (2) & \isa{{\normalsize{}If\,}\ \mbox{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}\ {\normalsize \,and\,}\ \mbox{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isacharprime}{\kern0pt}}\ {\normalsize \,then\,}\ v\ {\isacharequal}{\kern0pt}\ v{\isacharprime}{\kern0pt}{\isachardot}{\kern0pt}}
+ \end{tabular}
+ \end{theorem}
+
+ \begin{proof} Both by induction on the definition of \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}.
+ The second parts follows by a case analysis of \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isacharprime}{\kern0pt}} and
+ the first part.\qed
+ \end{proof}
+
+ \noindent
+ We claim that our \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v} relation captures the idea behind the four
+ informal POSIX rules shown in the Introduction: Consider for example the
+ rules \isa{P{\isacharplus}{\kern0pt}L} and \isa{P{\isacharplus}{\kern0pt}R} where the POSIX value for a string
+ and an alternative regular expression, that is \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}},
+ is specified---it is always a \isa{Left}-value, \emph{except} when the
+ string to be matched is not in the language of \isa{r\isactrlsub {\isadigit{1}}}; only then it
+ is a \isa{Right}-value (see the side-condition in \isa{P{\isacharplus}{\kern0pt}R}).
+ Interesting is also the rule for sequence regular expressions (\isa{PS}). The first two premises state that \isa{v\isactrlsub {\isadigit{1}}} and \isa{v\isactrlsub {\isadigit{2}}}
+ are the POSIX values for \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}} and \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}
+ respectively. Consider now the third premise and note that the POSIX value
+ of this rule should match the string \mbox{\isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}}}. According to the
+ Longest Match Rule, we want that the \isa{s\isactrlsub {\isadigit{1}}} is the longest initial
+ split of \mbox{\isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}}} such that \isa{s\isactrlsub {\isadigit{2}}} is still recognised
+ by \isa{r\isactrlsub {\isadigit{2}}}. Let us assume, contrary to the third premise, that there
+ \emph{exist} an \isa{s\isactrlsub {\isadigit{3}}} and \isa{s\isactrlsub {\isadigit{4}}} such that \isa{s\isactrlsub {\isadigit{2}}}
+ can be split up into a non-empty string \isa{s\isactrlsub {\isadigit{3}}} and a possibly empty
+ string \isa{s\isactrlsub {\isadigit{4}}}. Moreover the longer string \isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{3}}} can be
+ matched by \isa{r\isactrlsub {\isadigit{1}}} and the shorter \isa{s\isactrlsub {\isadigit{4}}} can still be
+ matched by \isa{r\isactrlsub {\isadigit{2}}}. In this case \isa{s\isactrlsub {\isadigit{1}}} would \emph{not} be the
+ longest initial split of \mbox{\isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}}} and therefore \isa{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}} cannot be a POSIX value for \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}.
+ The main point is that our side-condition ensures the Longest
+ Match Rule is satisfied.
+
+ A similar condition is imposed on the POSIX value in the \isa{P{\isasymstar}}-rule. Also there we want that \isa{s\isactrlsub {\isadigit{1}}} is the longest initial
+ split of \isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}} and furthermore the corresponding value
+ \isa{v} cannot be flattened to the empty string. In effect, we require
+ that in each ``iteration'' of the star, some non-empty substring needs to
+ be ``chipped'' away; only in case of the empty string we accept \isa{Stars\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} as the POSIX value. Indeed we can show that our POSIX values
+ are lexical values which exclude those \isa{Stars} that contain subvalues
+ that flatten to the empty string.
+
+ \begin{lemma}\label{LVposix}
+ \isa{{\normalsize{}If\,}\ {\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\ {\normalsize \,then\,}\ v\ {\isasymin}\ LV\ r\ s{\isachardot}{\kern0pt}}
+ \end{lemma}
+
+ \begin{proof}
+ By routine induction on \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}.\qed
+ \end{proof}
+
+ \noindent
+ Next is the lemma that shows the function \isa{mkeps} calculates
+ the POSIX value for the empty string and a nullable regular expression.
+
+ \begin{lemma}\label{lemmkeps}
+ \isa{{\normalsize{}If\,}\ nullable\ r\ {\normalsize \,then\,}\ {\isacharparenleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ mkeps\ r{\isachardot}{\kern0pt}}
+ \end{lemma}
+
+ \begin{proof}
+ By routine induction on \isa{r}.\qed
+ \end{proof}
+
+ \noindent
+ The central lemma for our POSIX relation is that the \isa{inj}-function
+ preserves POSIX values.
+
+ \begin{lemma}\label{Posix2}
+ \isa{{\normalsize{}If\,}\ {\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\ {\normalsize \,then\,}\ {\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ inj\ r\ c\ v{\isachardot}{\kern0pt}}
+ \end{lemma}
+
+ \begin{proof}
+ By induction on \isa{r}. We explain two cases.
+
+ \begin{itemize}
+ \item[$\bullet$] Case \isa{r\ {\isacharequal}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}}. There are
+ two subcases, namely \isa{{\isacharparenleft}{\kern0pt}a{\isacharparenright}{\kern0pt}} \mbox{\isa{v\ {\isacharequal}{\kern0pt}\ Left\ v{\isacharprime}{\kern0pt}}} and \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isacharprime}{\kern0pt}}; and \isa{{\isacharparenleft}{\kern0pt}b{\isacharparenright}{\kern0pt}} \isa{v\ {\isacharequal}{\kern0pt}\ Right\ v{\isacharprime}{\kern0pt}}, \isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}} and \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isacharprime}{\kern0pt}}. In \isa{{\isacharparenleft}{\kern0pt}a{\isacharparenright}{\kern0pt}} we
+ know \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isacharprime}{\kern0pt}}, from which we can infer \isa{{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ inj\ r\isactrlsub {\isadigit{1}}\ c\ v{\isacharprime}{\kern0pt}} by induction hypothesis and hence \isa{{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ inj\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ c\ {\isacharparenleft}{\kern0pt}Left\ v{\isacharprime}{\kern0pt}{\isacharparenright}{\kern0pt}} as needed. Similarly
+ in subcase \isa{{\isacharparenleft}{\kern0pt}b{\isacharparenright}{\kern0pt}} where, however, in addition we have to use
+ Proposition~\ref{derprop}(2) in order to infer \isa{c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}} from \isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}}.\smallskip
+
+ \item[$\bullet$] Case \isa{r\ {\isacharequal}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}}. There are three subcases:
+
+ \begin{quote}
+ \begin{description}
+ \item[\isa{{\isacharparenleft}{\kern0pt}a{\isacharparenright}{\kern0pt}}] \isa{v\ {\isacharequal}{\kern0pt}\ Left\ {\isacharparenleft}{\kern0pt}Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} and \isa{nullable\ r\isactrlsub {\isadigit{1}}}
+ \item[\isa{{\isacharparenleft}{\kern0pt}b{\isacharparenright}{\kern0pt}}] \isa{v\ {\isacharequal}{\kern0pt}\ Right\ v\isactrlsub {\isadigit{1}}} and \isa{nullable\ r\isactrlsub {\isadigit{1}}}
+ \item[\isa{{\isacharparenleft}{\kern0pt}c{\isacharparenright}{\kern0pt}}] \isa{v\ {\isacharequal}{\kern0pt}\ Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}} and \isa{{\isasymnot}\ nullable\ r\isactrlsub {\isadigit{1}}}
+ \end{description}
+ \end{quote}
+
+ \noindent For \isa{{\isacharparenleft}{\kern0pt}a{\isacharparenright}{\kern0pt}} we know \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{1}}} and
+ \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{2}}} as well as
+ %
+ \[\isa{{\isasymnexists}s\isactrlsub {\isadigit{3}}\ s\isactrlsub {\isadigit{4}}{\isachardot}{\kern0pt}a{\isachardot}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{3}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{4}}\ {\isacharequal}{\kern0pt}\ s\isactrlsub {\isadigit{2}}\ {\isasymand}\ s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{4}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\]
+
+ \noindent From the latter we can infer by Proposition~\ref{derprop}(2):
+ %
+ \[\isa{{\isasymnexists}s\isactrlsub {\isadigit{3}}\ s\isactrlsub {\isadigit{4}}{\isachardot}{\kern0pt}a{\isachardot}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{3}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{4}}\ {\isacharequal}{\kern0pt}\ s\isactrlsub {\isadigit{2}}\ {\isasymand}\ c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{4}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\]
+
+ \noindent We can use the induction hypothesis for \isa{r\isactrlsub {\isadigit{1}}} to obtain
+ \isa{{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ inj\ r\isactrlsub {\isadigit{1}}\ c\ v\isactrlsub {\isadigit{1}}}. Putting this all together allows us to infer
+ \isa{{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Seq\ {\isacharparenleft}{\kern0pt}inj\ r\isactrlsub {\isadigit{1}}\ c\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ v\isactrlsub {\isadigit{2}}}. The case \isa{{\isacharparenleft}{\kern0pt}c{\isacharparenright}{\kern0pt}}
+ is similar.
+
+ For \isa{{\isacharparenleft}{\kern0pt}b{\isacharparenright}{\kern0pt}} we know \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{1}}} and
+ \isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}. From the former
+ we have \isa{{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ inj\ r\isactrlsub {\isadigit{2}}\ c\ v\isactrlsub {\isadigit{1}}} by induction hypothesis
+ for \isa{r\isactrlsub {\isadigit{2}}}. From the latter we can infer
+ %
+ \[\isa{{\isasymnexists}s\isactrlsub {\isadigit{3}}\ s\isactrlsub {\isadigit{4}}{\isachardot}{\kern0pt}a{\isachardot}{\kern0pt}\ s\isactrlsub {\isadigit{3}}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{3}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{4}}\ {\isacharequal}{\kern0pt}\ c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s\ {\isasymand}\ s\isactrlsub {\isadigit{3}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymand}\ s\isactrlsub {\isadigit{4}}\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\]
+
+ \noindent By Lemma~\ref{lemmkeps} we know \isa{{\isacharparenleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ mkeps\ r\isactrlsub {\isadigit{1}}}
+ holds. Putting this all together, we can conclude with \isa{{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Seq\ {\isacharparenleft}{\kern0pt}mkeps\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}inj\ r\isactrlsub {\isadigit{2}}\ c\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}}, as required.
+
+ Finally suppose \isa{r\ {\isacharequal}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\isactrlsup {\isasymstar}}. This case is very similar to the
+ sequence case, except that we need to also ensure that \isa{{\isacharbar}{\kern0pt}inj\ r\isactrlsub {\isadigit{1}}\ c\ v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isasymnoteq}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}. This follows from \isa{{\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ inj\ r\isactrlsub {\isadigit{1}}\ c\ v\isactrlsub {\isadigit{1}}} (which in turn follows from \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{1}}} and the induction hypothesis).\qed
+ \end{itemize}
+ \end{proof}
+
+ \noindent
+ With Lemma~\ref{Posix2} in place, it is completely routine to establish
+ that the Sulzmann and Lu lexer satisfies our specification (returning
+ the null value \isa{None} iff the string is not in the language of the regular expression,
+ and returning a unique POSIX value iff the string \emph{is} in the language):
+
+ \begin{theorem}\mbox{}\smallskip\\\label{lexercorrect}
+ \begin{tabular}{ll}
+ (1) & \isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}} if and only if \isa{lexer\ r\ s\ {\isacharequal}{\kern0pt}\ None}\\
+ (2) & \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}} if and only if \isa{{\isasymexists}v{\isachardot}{\kern0pt}\ lexer\ r\ s\ {\isacharequal}{\kern0pt}\ Some\ v\ {\isasymand}\ {\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}\\
+ \end{tabular}
+ \end{theorem}
+
+ \begin{proof}
+ By induction on \isa{s} using Lemma~\ref{lemmkeps} and \ref{Posix2}.\qed
+ \end{proof}
+
+ \noindent In \textit{(2)} we further know by Theorem~\ref{posixdeterm} that the
+ value returned by the lexer must be unique. A simple corollary
+ of our two theorems is:
+
+ \begin{corollary}\mbox{}\smallskip\\\label{lexercorrectcor}
+ \begin{tabular}{ll}
+ (1) & \isa{lexer\ r\ s\ {\isacharequal}{\kern0pt}\ None} if and only if \isa{{\isasymnexists}v{\isachardot}{\kern0pt}a{\isachardot}{\kern0pt}\ {\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}\\
+ (2) & \isa{lexer\ r\ s\ {\isacharequal}{\kern0pt}\ Some\ v} if and only if \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}\\
+ \end{tabular}
+ \end{corollary}
+
+ \noindent This concludes our correctness proof. Note that we have
+ not changed the algorithm of Sulzmann and Lu,\footnote{All
+ deviations we introduced are harmless.} but introduced our own
+ specification for what a correct result---a POSIX value---should be.
+ In the next section we show that our specification coincides with
+ another one given by Okui and Suzuki using a different technique.%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{Ordering of Values according to Okui and Suzuki%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+While in the previous section we have defined POSIX values directly
+ in terms of a ternary relation (see inference rules in Figure~\ref{POSIXrules}),
+ Sulzmann and Lu took a different approach in \cite{Sulzmann2014}:
+ they introduced an ordering for values and identified POSIX values
+ as the maximal elements. An extended version of \cite{Sulzmann2014}
+ is available at the website of its first author; this includes more
+ details of their proofs, but which are evidently not in final form
+ yet. Unfortunately, we were not able to verify claims that their
+ ordering has properties such as being transitive or having maximal
+ elements.
+
+ Okui and Suzuki \cite{OkuiSuzuki2010,OkuiSuzukiTech} described
+ another ordering of values, which they use to establish the
+ correctness of their automata-based algorithm for POSIX matching.
+ Their ordering resembles some aspects of the one given by Sulzmann
+ and Lu, but overall is quite different. To begin with, Okui and
+ Suzuki identify POSIX values as minimal, rather than maximal,
+ elements in their ordering. A more substantial difference is that
+ the ordering by Okui and Suzuki uses \emph{positions} in order to
+ identify and compare subvalues. Positions are lists of natural
+ numbers. This allows them to quite naturally formalise the Longest
+ Match and Priority rules of the informal POSIX standard. Consider
+ for example the value \isa{v}
+
+ \begin{center}
+ \isa{v\ {\isasymequiv}\ Stars\ {\isacharbrackleft}{\kern0pt}Seq\ {\isacharparenleft}{\kern0pt}Char\ x{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}Char\ y{\isacharparenright}{\kern0pt}{\isacharcomma}{\kern0pt}\ Char\ z{\isacharbrackright}{\kern0pt}}
+ \end{center}
+
+ \noindent
+ At position \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{0}}{\isacharcomma}{\kern0pt}{\isadigit{1}}{\isacharbrackright}{\kern0pt}} of this value is the
+ subvalue \isa{Char\ y} and at position \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{1}}{\isacharbrackright}{\kern0pt}} the
+ subvalue \isa{Char\ z}. At the `root' position, or empty list
+ \isa{{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}}, is the whole value \isa{v}. Positions such as \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{0}}{\isacharcomma}{\kern0pt}{\isadigit{1}}{\isacharcomma}{\kern0pt}{\isadigit{0}}{\isacharbrackright}{\kern0pt}} or \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{2}}{\isacharbrackright}{\kern0pt}} are outside of \isa{v}. If it exists, the subvalue of \isa{v} at a position \isa{p}, written \isa{v\mbox{$\downharpoonleft$}\isactrlbsub p\isactrlesub }, can be recursively defined by
+
+ \begin{center}
+ \begin{tabular}{r@ {\hspace{0mm}}lcl}
+ \isa{v} & \isa{{\isasymdownharpoonleft}\isactrlbsub {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\isactrlesub } & \isa{{\isasymequiv}}& \isa{v}\\
+ \isa{Left\ v} & \isa{{\isasymdownharpoonleft}\isactrlbsub {\isadigit{0}}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}ps\isactrlesub } & \isa{{\isasymequiv}}& \isa{v\mbox{$\downharpoonleft$}\isactrlbsub ps\isactrlesub }\\
+ \isa{Right\ v} & \isa{{\isasymdownharpoonleft}\isactrlbsub {\isadigit{1}}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}ps\isactrlesub } & \isa{{\isasymequiv}} &
+ \isa{v\mbox{$\downharpoonleft$}\isactrlbsub ps\isactrlesub }\\
+ \isa{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}} & \isa{{\isasymdownharpoonleft}\isactrlbsub {\isadigit{0}}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}ps\isactrlesub } & \isa{{\isasymequiv}} &
+ \isa{v\isactrlsub {\isadigit{1}}\mbox{$\downharpoonleft$}\isactrlbsub ps\isactrlesub } \\
+ \isa{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}} & \isa{{\isasymdownharpoonleft}\isactrlbsub {\isadigit{1}}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}ps\isactrlesub }
+ & \isa{{\isasymequiv}} &
+ \isa{v\isactrlsub {\isadigit{2}}\mbox{$\downharpoonleft$}\isactrlbsub ps\isactrlesub } \\
+ \isa{Stars\ vs} & \isa{{\isasymdownharpoonleft}\isactrlbsub n{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}ps\isactrlesub } & \isa{{\isasymequiv}}& \isa{vs\ensuremath{_{[\mathit{n}]}}\mbox{$\downharpoonleft$}\isactrlbsub ps\isactrlesub }\\
+ \end{tabular}
+ \end{center}
+
+ \noindent In the last clause we use Isabelle's notation \isa{vs\ensuremath{_{[\mathit{n}]}}} for the
+ \isa{n}th element in a list. The set of positions inside a value \isa{v},
+ written \isa{Pos\ v}, is given by
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{Pos\ {\isacharparenleft}{\kern0pt}Empty{\isacharparenright}{\kern0pt}} & \isa{{\isasymequiv}} & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}}\\
+ \isa{Pos\ {\isacharparenleft}{\kern0pt}Char\ c{\isacharparenright}{\kern0pt}} & \isa{{\isasymequiv}} & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}}\\
+ \isa{Pos\ {\isacharparenleft}{\kern0pt}Left\ v{\isacharparenright}{\kern0pt}} & \isa{{\isasymequiv}} & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isasymunion}\ {\isacharbraceleft}{\kern0pt}{\isadigit{0}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\ \mbox{\boldmath$\mid$}\ ps\ {\isasymin}\ Pos\ v{\isacharbraceright}{\kern0pt}}\\
+ \isa{Pos\ {\isacharparenleft}{\kern0pt}Right\ v{\isacharparenright}{\kern0pt}} & \isa{{\isasymequiv}} & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isasymunion}\ {\isacharbraceleft}{\kern0pt}{\isadigit{1}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\ \mbox{\boldmath$\mid$}\ ps\ {\isasymin}\ Pos\ v{\isacharbraceright}{\kern0pt}}\\
+ \isa{Pos\ {\isacharparenleft}{\kern0pt}Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}
+ & \isa{{\isasymequiv}}
+ & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isasymunion}\ {\isacharbraceleft}{\kern0pt}{\isadigit{0}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\ \mbox{\boldmath$\mid$}\ ps\ {\isasymin}\ Pos\ v\isactrlsub {\isadigit{1}}{\isacharbraceright}{\kern0pt}\ {\isasymunion}\ {\isacharbraceleft}{\kern0pt}{\isadigit{1}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\ \mbox{\boldmath$\mid$}\ ps\ {\isasymin}\ Pos\ v\isactrlsub {\isadigit{2}}{\isacharbraceright}{\kern0pt}}\\
+ \isa{Pos\ {\isacharparenleft}{\kern0pt}Stars\ vs{\isacharparenright}{\kern0pt}} & \isa{{\isasymequiv}} & \isa{{\isacharbraceleft}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isasymunion}\ {\isacharparenleft}{\kern0pt}{\isasymUnion}n\ {\isacharless}{\kern0pt}\ len\ vs\ {\isacharbraceleft}{\kern0pt}n\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\ \mbox{\boldmath$\mid$}\ ps\ {\isasymin}\ Pos\ vs\ensuremath{_{[\mathit{n}]}}{\isacharbraceright}{\kern0pt}{\isacharparenright}{\kern0pt}}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent
+ whereby \isa{len} in the last clause stands for the length of a list. Clearly
+ for every position inside a value there exists a subvalue at that position.
+
+
+ To help understanding the ordering of Okui and Suzuki, consider again
+ the earlier value
+ \isa{v} and compare it with the following \isa{w}:
+
+ \begin{center}
+ \begin{tabular}{l}
+ \isa{v\ {\isasymequiv}\ Stars\ {\isacharbrackleft}{\kern0pt}Seq\ {\isacharparenleft}{\kern0pt}Char\ x{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}Char\ y{\isacharparenright}{\kern0pt}{\isacharcomma}{\kern0pt}\ Char\ z{\isacharbrackright}{\kern0pt}}\\
+ \isa{w\ {\isasymequiv}\ Stars\ {\isacharbrackleft}{\kern0pt}Char\ x{\isacharcomma}{\kern0pt}\ Char\ y{\isacharcomma}{\kern0pt}\ Char\ z{\isacharbrackright}{\kern0pt}}
+ \end{tabular}
+ \end{center}
+
+ \noindent Both values match the string \isa{xyz}, that means if
+ we flatten these values at their respective root position, we obtain
+ \isa{xyz}. However, at position \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{0}}{\isacharbrackright}{\kern0pt}}, \isa{v} matches
+ \isa{xy} whereas \isa{w} matches only the shorter \isa{x}. So
+ according to the Longest Match Rule, we should prefer \isa{v},
+ rather than \isa{w} as POSIX value for string \isa{xyz} (and
+ corresponding regular expression). In order to
+ formalise this idea, Okui and Suzuki introduce a measure for
+ subvalues at position \isa{p}, called the \emph{norm} of \isa{v}
+ at position \isa{p}. We can define this measure in Isabelle as an
+ integer as follows
+
+ \begin{center}
+ \isa{{\isasymparallel}v{\isasymparallel}\isactrlbsub p\isactrlesub \ {\isasymequiv}\ \textrm{if}\ p\ {\isasymin}\ Pos\ v\ \textrm{then}\ len\ {\isacharbar}{\kern0pt}v\mbox{$\downharpoonleft$}\isactrlbsub p\isactrlesub {\isacharbar}{\kern0pt}\ \textrm{else}\ {\isacharminus}{\kern0pt}\ {\isadigit{1}}}
+ \end{center}
+
+ \noindent where we take the length of the flattened value at
+ position \isa{p}, provided the position is inside \isa{v}; if
+ not, then the norm is \isa{{\isacharminus}{\kern0pt}{\isadigit{1}}}. The default for outside
+ positions is crucial for the POSIX requirement of preferring a
+ \isa{Left}-value over a \isa{Right}-value (if they can match the
+ same string---see the Priority Rule from the Introduction). For this
+ consider
+
+ \begin{center}
+ \isa{v\ {\isasymequiv}\ Left\ {\isacharparenleft}{\kern0pt}Char\ x{\isacharparenright}{\kern0pt}} \qquad and \qquad \isa{w\ {\isasymequiv}\ Right\ {\isacharparenleft}{\kern0pt}Char\ x{\isacharparenright}{\kern0pt}}
+ \end{center}
+
+ \noindent Both values match \isa{x}. At position \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{0}}{\isacharbrackright}{\kern0pt}}
+ the norm of \isa{v} is \isa{{\isadigit{1}}} (the subvalue matches \isa{x}),
+ but the norm of \isa{w} is \isa{{\isacharminus}{\kern0pt}{\isadigit{1}}} (the position is outside
+ \isa{w} according to how we defined the `inside' positions of
+ \isa{Left}- and \isa{Right}-values). Of course at position
+ \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{1}}{\isacharbrackright}{\kern0pt}}, the norms \isa{{\isasymparallel}v{\isasymparallel}\isactrlbsub {\isacharbrackleft}{\kern0pt}{\isadigit{1}}{\isacharbrackright}{\kern0pt}\isactrlesub } and \isa{{\isasymparallel}w{\isasymparallel}\isactrlbsub {\isacharbrackleft}{\kern0pt}{\isadigit{1}}{\isacharbrackright}{\kern0pt}\isactrlesub } are reversed, but the point is that subvalues
+ will be analysed according to lexicographically ordered
+ positions. According to this ordering, the position \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{0}}{\isacharbrackright}{\kern0pt}}
+ takes precedence over \isa{{\isacharbrackleft}{\kern0pt}{\isadigit{1}}{\isacharbrackright}{\kern0pt}} and thus also \isa{v} will be
+ preferred over \isa{w}. The lexicographic ordering of positions, written
+ \isa{\underline{\hspace{2mm}}\ {\isasymprec}\isactrlbsub lex\isactrlesub \ \underline{\hspace{2mm}}}, can be conveniently formalised
+ by three inference rules
+
+ \begin{center}
+ \begin{tabular}{ccc}
+ \isa{\mbox{}\inferrule{\mbox{}}{\mbox{{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ {\isasymprec}\isactrlbsub lex\isactrlesub \ p\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps}}}\hspace{1cm} &
+ \isa{\mbox{}\inferrule{\mbox{p\isactrlsub {\isadigit{1}}\ {\isacharless}{\kern0pt}\ p\isactrlsub {\isadigit{2}}}}{\mbox{p\isactrlsub {\isadigit{1}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\isactrlsub {\isadigit{1}}\ {\isasymprec}\isactrlbsub lex\isactrlesub \ p\isactrlsub {\isadigit{2}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\isactrlsub {\isadigit{2}}}}}\hspace{1cm} &
+ \isa{\mbox{}\inferrule{\mbox{ps\isactrlsub {\isadigit{1}}\ {\isasymprec}\isactrlbsub lex\isactrlesub \ ps\isactrlsub {\isadigit{2}}}}{\mbox{p\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\isactrlsub {\isadigit{1}}\ {\isasymprec}\isactrlbsub lex\isactrlesub \ p\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}ps\isactrlsub {\isadigit{2}}}}}
+ \end{tabular}
+ \end{center}
+
+ With the norm and lexicographic order in place,
+ we can state the key definition of Okui and Suzuki
+ \cite{OkuiSuzuki2010}: a value \isa{v\isactrlsub {\isadigit{1}}} is \emph{smaller at position \isa{p}} than
+ \isa{v\isactrlsub {\isadigit{2}}}, written \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\isactrlbsub p\isactrlesub \ v\isactrlsub {\isadigit{2}}},
+ if and only if $(i)$ the norm at position \isa{p} is
+ greater in \isa{v\isactrlsub {\isadigit{1}}} (that is the string \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}\mbox{$\downharpoonleft$}\isactrlbsub p\isactrlesub {\isacharbar}{\kern0pt}} is longer
+ than \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}\mbox{$\downharpoonleft$}\isactrlbsub p\isactrlesub {\isacharbar}{\kern0pt}}) and $(ii)$ all subvalues at
+ positions that are inside \isa{v\isactrlsub {\isadigit{1}}} or \isa{v\isactrlsub {\isadigit{2}}} and that are
+ lexicographically smaller than \isa{p}, we have the same norm, namely
+
+ \begin{center}
+ \begin{tabular}{c}
+ \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\isactrlbsub p\isactrlesub \ v\isactrlsub {\isadigit{2}}}
+ \isa{{\isasymequiv}}
+ $\begin{cases}
+ (i) & \isa{{\isasymparallel}v\isactrlsub {\isadigit{2}}{\isasymparallel}\isactrlbsub p\isactrlesub \ {\isacharless}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{1}}{\isasymparallel}\isactrlbsub p\isactrlesub } \quad\text{and}\smallskip \\
+ (ii) & \isa{{\isasymforall}q{\isasymin}Pos\ v\isactrlsub {\isadigit{1}}\ {\isasymunion}\ Pos\ v\isactrlsub {\isadigit{2}}{\isachardot}{\kern0pt}\ q\ {\isasymprec}\isactrlbsub lex\isactrlesub \ p\ {\isasymlongrightarrow}\ {\isasymparallel}v\isactrlsub {\isadigit{1}}{\isasymparallel}\isactrlbsub q\isactrlesub \ {\isacharequal}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{2}}{\isasymparallel}\isactrlbsub q\isactrlesub }
+ \end{cases}$
+ \end{tabular}
+ \end{center}
+
+ \noindent The position \isa{p} in this definition acts as the
+ \emph{first distinct position} of \isa{v\isactrlsub {\isadigit{1}}} and \isa{v\isactrlsub {\isadigit{2}}}, where both values match strings of different length
+ \cite{OkuiSuzuki2010}. Since at \isa{p} the values \isa{v\isactrlsub {\isadigit{1}}} and \isa{v\isactrlsub {\isadigit{2}}} match different strings, the
+ ordering is irreflexive. Derived from the definition above
+ are the following two orderings:
+
+ \begin{center}
+ \begin{tabular}{l}
+ \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}\ {\isasymequiv}\ {\isasymexists}p{\isachardot}{\kern0pt}\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\isactrlbsub p\isactrlesub \ v\isactrlsub {\isadigit{2}}}\\
+ \isa{v\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{2}}\ {\isasymequiv}\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}\ {\isasymor}\ v\isactrlsub {\isadigit{1}}\ {\isacharequal}{\kern0pt}\ v\isactrlsub {\isadigit{2}}}
+ \end{tabular}
+ \end{center}
+
+ While we encountered a number of obstacles for establishing properties like
+ transitivity for the ordering of Sulzmann and Lu (and which we failed
+ to overcome), it is relatively straightforward to establish this
+ property for the orderings
+ \isa{\underline{\hspace{2mm}}\ {\isasymprec}\ \underline{\hspace{2mm}}} and \isa{\underline{\hspace{2mm}}\ \mbox{$\preccurlyeq$}\ \underline{\hspace{2mm}}}
+ by Okui and Suzuki.
+
+ \begin{lemma}[Transitivity]\label{transitivity}
+ \isa{{\normalsize{}If\,}\ \mbox{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}}\ {\normalsize \,and\,}\ \mbox{v\isactrlsub {\isadigit{2}}\ {\isasymprec}\ v\isactrlsub {\isadigit{3}}}\ {\normalsize \,then\,}\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{3}}{\isachardot}{\kern0pt}}
+ \end{lemma}
+
+ \begin{proof} From the assumption we obtain two positions \isa{p}
+ and \isa{q}, where the values \isa{v\isactrlsub {\isadigit{1}}} and \isa{v\isactrlsub {\isadigit{2}}} (respectively \isa{v\isactrlsub {\isadigit{2}}} and \isa{v\isactrlsub {\isadigit{3}}}) are `distinct'. Since \isa{{\isasymprec}\isactrlbsub lex\isactrlesub } is trichotomous, we need to consider
+ three cases, namely \isa{p\ {\isacharequal}{\kern0pt}\ q}, \isa{p\ {\isasymprec}\isactrlbsub lex\isactrlesub \ q} and
+ \isa{q\ {\isasymprec}\isactrlbsub lex\isactrlesub \ p}. Let us look at the first case. Clearly
+ \isa{{\isasymparallel}v\isactrlsub {\isadigit{2}}{\isasymparallel}\isactrlbsub p\isactrlesub \ {\isacharless}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{1}}{\isasymparallel}\isactrlbsub p\isactrlesub } and \isa{{\isasymparallel}v\isactrlsub {\isadigit{3}}{\isasymparallel}\isactrlbsub p\isactrlesub \ {\isacharless}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{2}}{\isasymparallel}\isactrlbsub p\isactrlesub } imply \isa{{\isasymparallel}v\isactrlsub {\isadigit{3}}{\isasymparallel}\isactrlbsub p\isactrlesub \ {\isacharless}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{1}}{\isasymparallel}\isactrlbsub p\isactrlesub }. It remains to show
+ that for a \isa{p{\isacharprime}{\kern0pt}\ {\isasymin}\ Pos\ v\isactrlsub {\isadigit{1}}\ {\isasymunion}\ Pos\ v\isactrlsub {\isadigit{3}}}
+ with \isa{p{\isacharprime}{\kern0pt}\ {\isasymprec}\isactrlbsub lex\isactrlesub \ p} that \isa{{\isasymparallel}v\isactrlsub {\isadigit{1}}{\isasymparallel}\isactrlbsub p{\isacharprime}{\kern0pt}\isactrlesub \ {\isacharequal}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{3}}{\isasymparallel}\isactrlbsub p{\isacharprime}{\kern0pt}\isactrlesub } holds. Suppose \isa{p{\isacharprime}{\kern0pt}\ {\isasymin}\ Pos\ v\isactrlsub {\isadigit{1}}}, then we can infer from the first assumption that \isa{{\isasymparallel}v\isactrlsub {\isadigit{1}}{\isasymparallel}\isactrlbsub p{\isacharprime}{\kern0pt}\isactrlesub \ {\isacharequal}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{2}}{\isasymparallel}\isactrlbsub p{\isacharprime}{\kern0pt}\isactrlesub }. But this means
+ that \isa{p{\isacharprime}{\kern0pt}} must be in \isa{Pos\ v\isactrlsub {\isadigit{2}}} too (the norm
+ cannot be \isa{{\isacharminus}{\kern0pt}{\isadigit{1}}} given \isa{p{\isacharprime}{\kern0pt}\ {\isasymin}\ Pos\ v\isactrlsub {\isadigit{1}}}).
+ Hence we can use the second assumption and
+ infer \isa{{\isasymparallel}v\isactrlsub {\isadigit{2}}{\isasymparallel}\isactrlbsub p{\isacharprime}{\kern0pt}\isactrlesub \ {\isacharequal}{\kern0pt}\ {\isasymparallel}v\isactrlsub {\isadigit{3}}{\isasymparallel}\isactrlbsub p{\isacharprime}{\kern0pt}\isactrlesub },
+ which concludes this case with \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{3}}}. The reasoning in the other cases is similar.\qed
+ \end{proof}
+
+ \noindent
+ The proof for $\preccurlyeq$ is similar and omitted.
+ It is also straightforward to show that \isa{{\isasymprec}} and
+ $\preccurlyeq$ are partial orders. Okui and Suzuki furthermore show that they
+ are linear orderings for lexical values \cite{OkuiSuzuki2010} of a given
+ regular expression and given string, but we have not formalised this in Isabelle. It is
+ not essential for our results. What we are going to show below is
+ that for a given \isa{r} and \isa{s}, the orderings have a unique
+ minimal element on the set \isa{LV\ r\ s}, which is the POSIX value
+ we defined in the previous section. We start with two properties that
+ show how the length of a flattened value relates to the \isa{{\isasymprec}}-ordering.
+
+ \begin{proposition}\mbox{}\smallskip\\\label{ordlen}
+ \begin{tabular}{@ {}ll}
+ (1) &
+ \isa{{\normalsize{}If\,}\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}\ {\normalsize \,then\,}\ len\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}\ {\isasymle}\ len\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}{\isachardot}{\kern0pt}}\\
+ (2) &
+ \isa{{\normalsize{}If\,}\ len\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}\ {\isacharless}{\kern0pt}\ len\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\normalsize \,then\,}\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}{\isachardot}{\kern0pt}}
+ \end{tabular}
+ \end{proposition}
+
+ \noindent Both properties follow from the definition of the ordering. Note that
+ \textit{(2)} entails that a value, say \isa{v\isactrlsub {\isadigit{2}}}, whose underlying
+ string is a strict prefix of another flattened value, say \isa{v\isactrlsub {\isadigit{1}}}, then
+ \isa{v\isactrlsub {\isadigit{1}}} must be smaller than \isa{v\isactrlsub {\isadigit{2}}}. For our proofs it
+ will be useful to have the following properties---in each case the underlying strings
+ of the compared values are the same:
+
+ \begin{proposition}\mbox{}\smallskip\\\label{ordintros}
+ \begin{tabular}{ll}
+ \textit{(1)} &
+ \isa{{\normalsize{}If\,}\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}\ {\normalsize \,then\,}\ Left\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ Right\ v\isactrlsub {\isadigit{2}}{\isachardot}{\kern0pt}}\\
+ \textit{(2)} & If
+ \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} \;then\;
+ \isa{Left\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ Left\ v\isactrlsub {\isadigit{2}}} \;iff\;
+ \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}}\\
+ \textit{(3)} & If
+ \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} \;then\;
+ \isa{Right\ v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ Right\ v\isactrlsub {\isadigit{2}}} \;iff\;
+ \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}}\\
+ \textit{(4)} & If
+ \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}w\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} \;then\;
+ \isa{Seq\ v\ v\isactrlsub {\isadigit{2}}\ {\isasymprec}\ Seq\ v\ w\isactrlsub {\isadigit{2}}} \;iff\;
+ \isa{v\isactrlsub {\isadigit{2}}\ {\isasymprec}\ w\isactrlsub {\isadigit{2}}}\\
+ \textit{(5)} & If
+ \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharat}{\kern0pt}\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}w\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharat}{\kern0pt}\ {\isacharbar}{\kern0pt}w\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} \;and\;
+ \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ w\isactrlsub {\isadigit{1}}} \;then\;
+ \isa{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}\ {\isasymprec}\ Seq\ w\isactrlsub {\isadigit{1}}\ w\isactrlsub {\isadigit{2}}}\\
+ \textit{(6)} & If
+ \isa{{\isacharbar}{\kern0pt}vs\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}vs\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} \;then\;
+ \isa{Stars\ {\isacharparenleft}{\kern0pt}vs\ {\isacharat}{\kern0pt}\ vs\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymprec}\ Stars\ {\isacharparenleft}{\kern0pt}vs\ {\isacharat}{\kern0pt}\ vs\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} \;iff\;
+ \isa{Stars\ vs\isactrlsub {\isadigit{1}}\ {\isasymprec}\ Stars\ vs\isactrlsub {\isadigit{2}}}\\
+
+ \textit{(7)} & If
+ \isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{1}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} \;and\;
+ \isa{v\isactrlsub {\isadigit{1}}\ {\isasymprec}\ v\isactrlsub {\isadigit{2}}} \;then\;
+ \isa{Stars\ {\isacharparenleft}{\kern0pt}v\isactrlsub {\isadigit{1}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isasymprec}\ Stars\ {\isacharparenleft}{\kern0pt}v\isactrlsub {\isadigit{2}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \end{tabular}
+ \end{proposition}
+
+ \noindent One might prefer that statements \textit{(4)} and \textit{(5)}
+ (respectively \textit{(6)} and \textit{(7)})
+ are combined into a single \textit{iff}-statement (like the ones for \isa{Left} and \isa{Right}). Unfortunately this cannot be done easily: such
+ a single statement would require an additional assumption about the
+ two values \isa{Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}} and \isa{Seq\ w\isactrlsub {\isadigit{1}}\ w\isactrlsub {\isadigit{2}}}
+ being inhabited by the same regular expression. The
+ complexity of the proofs involved seems to not justify such a
+ `cleaner' single statement. The statements given are just the properties that
+ allow us to establish our theorems without any difficulty. The proofs
+ for Proposition~\ref{ordintros} are routine.
+
+
+ Next we establish how Okui and Suzuki's orderings relate to our
+ definition of POSIX values. Given a \isa{POSIX} value \isa{v\isactrlsub {\isadigit{1}}}
+ for \isa{r} and \isa{s}, then any other lexical value \isa{v\isactrlsub {\isadigit{2}}} in \isa{LV\ r\ s} is greater or equal than \isa{v\isactrlsub {\isadigit{1}}}, namely:
+
+
+ \begin{theorem}\label{orderone}
+ \isa{{\normalsize{}If\,}\ \mbox{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{1}}}\ {\normalsize \,and\,}\ \mbox{v\isactrlsub {\isadigit{2}}\ {\isasymin}\ LV\ r\ s}\ {\normalsize \,then\,}\ v\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{2}}{\isachardot}{\kern0pt}}
+ \end{theorem}
+
+ \begin{proof} By induction on our POSIX rules. By
+ Theorem~\ref{posixdeterm} and the definition of \isa{LV}, it is clear
+ that \isa{v\isactrlsub {\isadigit{1}}} and \isa{v\isactrlsub {\isadigit{2}}} have the same
+ underlying string \isa{s}. The three base cases are
+ straightforward: for example for \isa{v\isactrlsub {\isadigit{1}}\ {\isacharequal}{\kern0pt}\ Empty}, we have
+ that \isa{v\isactrlsub {\isadigit{2}}\ {\isasymin}\ LV\ \isactrlbold {\isadigit{1}}\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} must also be of the form
+ \mbox{\isa{v\isactrlsub {\isadigit{2}}\ {\isacharequal}{\kern0pt}\ Empty}}. Therefore we have \isa{v\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{2}}}. The inductive cases for
+ \isa{r} being of the form \isa{r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}} and
+ \isa{r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}} are as follows:
+
+
+ \begin{itemize}
+
+ \item[$\bullet$] Case \isa{P{\isacharplus}{\kern0pt}L} with \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Left\ w\isactrlsub {\isadigit{1}}}: In this case the value
+ \isa{v\isactrlsub {\isadigit{2}}} is either of the
+ form \isa{Left\ w\isactrlsub {\isadigit{2}}} or \isa{Right\ w\isactrlsub {\isadigit{2}}}. In the
+ latter case we can immediately conclude with \mbox{\isa{v\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{2}}}} since a \isa{Left}-value with the
+ same underlying string \isa{s} is always smaller than a
+ \isa{Right}-value by Proposition~\ref{ordintros}\textit{(1)}.
+ In the former case we have \isa{w\isactrlsub {\isadigit{2}}\ {\isasymin}\ LV\ r\isactrlsub {\isadigit{1}}\ s} and can use the induction hypothesis to infer
+ \isa{w\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ w\isactrlsub {\isadigit{2}}}. Because \isa{w\isactrlsub {\isadigit{1}}} and \isa{w\isactrlsub {\isadigit{2}}} have the same underlying string
+ \isa{s}, we can conclude with \isa{Left\ w\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ Left\ w\isactrlsub {\isadigit{2}}} using
+ Proposition~\ref{ordintros}\textit{(2)}.\smallskip
+
+ \item[$\bullet$] Case \isa{P{\isacharplus}{\kern0pt}R} with \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Right\ w\isactrlsub {\isadigit{1}}}: This case similar to the previous
+ case, except that we additionally know \isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}}. This is needed when \isa{v\isactrlsub {\isadigit{2}}} is of the form
+ \mbox{\isa{Left\ w\isactrlsub {\isadigit{2}}}}. Since \mbox{\isa{{\isacharbar}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}w\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}} \isa{{\isacharequal}{\kern0pt}\ s}} and \isa{w\isactrlsub {\isadigit{2}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{1}}}, we can derive a contradiction for \mbox{\isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}}} using
+ Proposition~\ref{inhabs}. So also in this case \mbox{\isa{v\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{2}}}}.\smallskip
+
+ \item[$\bullet$] Case \isa{PS} with \isa{{\isacharparenleft}{\kern0pt}s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Seq\ w\isactrlsub {\isadigit{1}}\ w\isactrlsub {\isadigit{2}}}: We can assume \isa{v\isactrlsub {\isadigit{2}}\ {\isacharequal}{\kern0pt}\ Seq\ u\isactrlsub {\isadigit{1}}\ u\isactrlsub {\isadigit{2}}} with \isa{u\isactrlsub {\isadigit{1}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{1}}} and \mbox{\isa{u\isactrlsub {\isadigit{2}}\ {\isacharcolon}{\kern0pt}\ r\isactrlsub {\isadigit{2}}}}. We have \isa{s\isactrlsub {\isadigit{1}}\ {\isacharat}{\kern0pt}\ s\isactrlsub {\isadigit{2}}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}u\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}\ {\isacharat}{\kern0pt}\ {\isacharbar}{\kern0pt}u\isactrlsub {\isadigit{2}}{\isacharbar}{\kern0pt}}. By the side-condition of the
+ \isa{PS}-rule we know that either \isa{s\isactrlsub {\isadigit{1}}\ {\isacharequal}{\kern0pt}\ {\isacharbar}{\kern0pt}u\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}} or that \isa{{\isacharbar}{\kern0pt}u\isactrlsub {\isadigit{1}}{\isacharbar}{\kern0pt}} is a strict prefix of
+ \isa{s\isactrlsub {\isadigit{1}}}. In the latter case we can infer \isa{w\isactrlsub {\isadigit{1}}\ {\isasymprec}\ u\isactrlsub {\isadigit{1}}} by
+ Proposition~\ref{ordlen}\textit{(2)} and from this \isa{v\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{2}}} by Proposition~\ref{ordintros}\textit{(5)}
+ (as noted above \isa{v\isactrlsub {\isadigit{1}}} and \isa{v\isactrlsub {\isadigit{2}}} must have the
+ same underlying string).
+ In the former case we know
+ \isa{u\isactrlsub {\isadigit{1}}\ {\isasymin}\ LV\ r\isactrlsub {\isadigit{1}}\ s\isactrlsub {\isadigit{1}}} and \isa{u\isactrlsub {\isadigit{2}}\ {\isasymin}\ LV\ r\isactrlsub {\isadigit{2}}\ s\isactrlsub {\isadigit{2}}}. With this we can use the
+ induction hypotheses to infer \isa{w\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ u\isactrlsub {\isadigit{1}}} and \isa{w\isactrlsub {\isadigit{2}}\ \mbox{$\preccurlyeq$}\ u\isactrlsub {\isadigit{2}}}. By
+ Proposition~\ref{ordintros}\textit{(4,5)} we can again infer
+ \isa{v\isactrlsub {\isadigit{1}}\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{2}}}.
+
+ \end{itemize}
+
+ \noindent The case for \isa{P{\isasymstar}} is similar to the \isa{PS}-case and omitted.\qed
+ \end{proof}
+
+ \noindent This theorem shows that our \isa{POSIX} value for a
+ regular expression \isa{r} and string \isa{s} is in fact a
+ minimal element of the values in \isa{LV\ r\ s}. By
+ Proposition~\ref{ordlen}\textit{(2)} we also know that any value in
+ \isa{LV\ r\ s{\isacharprime}{\kern0pt}}, with \isa{s{\isacharprime}{\kern0pt}} being a strict prefix, cannot be
+ smaller than \isa{v\isactrlsub {\isadigit{1}}}. The next theorem shows the
+ opposite---namely any minimal element in \isa{LV\ r\ s} must be a
+ \isa{POSIX} value. This can be established by induction on \isa{r}, but the proof can be drastically simplified by using the fact
+ from the previous section about the existence of a \isa{POSIX} value
+ whenever a string \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}}.
+
+
+ \begin{theorem}
+ \isa{{\normalsize{}If\,}\ \mbox{v\isactrlsub {\isadigit{1}}\ {\isasymin}\ LV\ r\ s}\ {\normalsize \,and\,}\ \mbox{{\isasymforall}v\isactrlsub {\isadigit{2}}{\isasymin}LV\ r\ s{\isachardot}{\kern0pt}\ v\isactrlsub {\isadigit{2}}\ \mbox{$\not\prec$}\ v\isactrlsub {\isadigit{1}}}\ {\normalsize \,then\,}\ {\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub {\isadigit{1}}{\isachardot}{\kern0pt}}
+ \end{theorem}
+
+ \begin{proof}
+ If \isa{v\isactrlsub {\isadigit{1}}\ {\isasymin}\ LV\ r\ s} then
+ \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}} by Proposition~\ref{inhabs}. Hence by Theorem~\ref{lexercorrect}(2)
+ there exists a
+ \isa{POSIX} value \isa{v\isactrlsub P} with \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\isactrlsub P}
+ and by Lemma~\ref{LVposix} we also have \mbox{\isa{v\isactrlsub P\ {\isasymin}\ LV\ r\ s}}.
+ By Theorem~\ref{orderone} we therefore have
+ \isa{v\isactrlsub P\ \mbox{$\preccurlyeq$}\ v\isactrlsub {\isadigit{1}}}. If \isa{v\isactrlsub P\ {\isacharequal}{\kern0pt}\ v\isactrlsub {\isadigit{1}}} then
+ we are done. Otherwise we have \isa{v\isactrlsub P\ {\isasymprec}\ v\isactrlsub {\isadigit{1}}}, which
+ however contradicts the second assumption about \isa{v\isactrlsub {\isadigit{1}}} being the smallest
+ element in \isa{LV\ r\ s}. So we are done in this case too.\qed
+ \end{proof}
+
+ \noindent
+ From this we can also show
+ that if \isa{LV\ r\ s} is non-empty (or equivalently \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}}) then
+ it has a unique minimal element:
+
+ \begin{corollary}
+ \isa{{\normalsize{}If\,}\ LV\ r\ s\ {\isasymnoteq}\ {\isasymemptyset}\ {\normalsize \,then\,}\ {\isasymexists}{\isacharbang}{\kern0pt}vmin{\isachardot}{\kern0pt}\ vmin\ {\isasymin}\ LV\ r\ s\ {\isasymand}\ {\isacharparenleft}{\kern0pt}{\isasymforall}v{\isasymin}LV\ r\ s{\isachardot}{\kern0pt}\ vmin\ \mbox{$\preccurlyeq$}\ v{\isacharparenright}{\kern0pt}{\isachardot}{\kern0pt}}
+ \end{corollary}
+
+
+
+ \noindent To sum up, we have shown that the (unique) minimal elements
+ of the ordering by Okui and Suzuki are exactly the \isa{POSIX}
+ values we defined inductively in Section~\ref{posixsec}. This provides
+ an independent confirmation that our ternary relation formalises the
+ informal POSIX rules.%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{Bitcoded Lexing%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+Incremental calculation of the value. To simplify the proof we first define the function
+\isa{flex} which calculates the ``iterated'' injection function. With this we can
+rewrite the lexer as
+
+\begin{center}
+\isa{lexer\ r\ s\ {\isacharequal}{\kern0pt}\ {\isacharparenleft}{\kern0pt}\textrm{if}\ nullable\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}s{\isacharparenright}{\kern0pt}\ \textrm{then}\ Some\ {\isacharparenleft}{\kern0pt}flex\ r\ id\ s\ {\isacharparenleft}{\kern0pt}mkeps\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}s{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ \textrm{else}\ None{\isacharparenright}{\kern0pt}}
+\end{center}%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{Optimisations%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+Derivatives as calculated by \Brz's method are usually more complex
+ regular expressions than the initial one; the result is that the
+ derivative-based matching and lexing algorithms are often abysmally slow.
+ However, various optimisations are possible, such as the simplifications
+ of \isa{\isactrlbold {\isadigit{0}}\ {\isacharplus}{\kern0pt}\ r}, \isa{r\ {\isacharplus}{\kern0pt}\ \isactrlbold {\isadigit{0}}}, \isa{\isactrlbold {\isadigit{1}}\ {\isasymcdot}\ r} and
+ \isa{r\ {\isasymcdot}\ \isactrlbold {\isadigit{1}}} to \isa{r}. These simplifications can speed up the
+ algorithms considerably, as noted in \cite{Sulzmann2014}. One of the
+ advantages of having a simple specification and correctness proof is that
+ the latter can be refined to prove the correctness of such simplification
+ steps. While the simplification of regular expressions according to
+ rules like
+
+ \begin{equation}\label{Simpl}
+ \begin{array}{lcllcllcllcl}
+ \isa{\isactrlbold {\isadigit{0}}\ {\isacharplus}{\kern0pt}\ r} & \isa{{\isasymRightarrow}} & \isa{r} \hspace{8mm}%\\
+ \isa{r\ {\isacharplus}{\kern0pt}\ \isactrlbold {\isadigit{0}}} & \isa{{\isasymRightarrow}} & \isa{r} \hspace{8mm}%\\
+ \isa{\isactrlbold {\isadigit{1}}\ {\isasymcdot}\ r} & \isa{{\isasymRightarrow}} & \isa{r} \hspace{8mm}%\\
+ \isa{r\ {\isasymcdot}\ \isactrlbold {\isadigit{1}}} & \isa{{\isasymRightarrow}} & \isa{r}
+ \end{array}
+ \end{equation}
+
+ \noindent is well understood, there is an obstacle with the POSIX value
+ calculation algorithm by Sulzmann and Lu: if we build a derivative regular
+ expression and then simplify it, we will calculate a POSIX value for this
+ simplified derivative regular expression, \emph{not} for the original (unsimplified)
+ derivative regular expression. Sulzmann and Lu \cite{Sulzmann2014} overcome this obstacle by
+ not just calculating a simplified regular expression, but also calculating
+ a \emph{rectification function} that ``repairs'' the incorrect value.
+
+ The rectification functions can be (slightly clumsily) implemented in
+ Isabelle/HOL as follows using some auxiliary functions:
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{F\isactrlbsub Right\isactrlesub \ f\ v} & $\dn$ & \isa{Right\ {\isacharparenleft}{\kern0pt}f\ v{\isacharparenright}{\kern0pt}}\\
+ \isa{F\isactrlbsub Left\isactrlesub \ f\ v} & $\dn$ & \isa{Left\ {\isacharparenleft}{\kern0pt}f\ v{\isacharparenright}{\kern0pt}}\\
+
+ \isa{F\isactrlbsub Alt\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}\ {\isacharparenleft}{\kern0pt}Right\ v{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{Right\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ v{\isacharparenright}{\kern0pt}}\\
+ \isa{F\isactrlbsub Alt\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}\ {\isacharparenleft}{\kern0pt}Left\ v{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{Left\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ v{\isacharparenright}{\kern0pt}}\\
+
+ \isa{F\isactrlbsub Seq{\isadigit{1}}\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}\ v} & $\dn$ & \isa{Seq\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ {\isacharparenleft}{\kern0pt}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ v{\isacharparenright}{\kern0pt}}\\
+ \isa{F\isactrlbsub Seq{\isadigit{2}}\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}\ v} & $\dn$ & \isa{Seq\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ v{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ {\isacharparenleft}{\kern0pt}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}}\\
+ \isa{F\isactrlbsub Seq\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}\ {\isacharparenleft}{\kern0pt}Seq\ v\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{Seq\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\medskip\\
+ %\end{tabular}
+ %
+ %\begin{tabular}{lcl}
+ \isa{simp\isactrlbsub Alt\isactrlesub \ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{0}}{\isacharcomma}{\kern0pt}\ \underline{\hspace{2mm}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F\isactrlbsub Right\isactrlesub \ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{simp\isactrlbsub Alt\isactrlesub \ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{0}}{\isacharcomma}{\kern0pt}\ \underline{\hspace{2mm}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ F\isactrlbsub Left\isactrlesub \ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}}\\
+ \isa{simp\isactrlbsub Alt\isactrlesub \ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F\isactrlbsub Alt\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{simp\isactrlbsub Seq\isactrlesub \ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F\isactrlbsub Seq{\isadigit{1}}\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{simp\isactrlbsub Seq\isactrlesub \ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}\isactrlbold {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ F\isactrlbsub Seq{\isadigit{2}}\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{simp\isactrlbsub Seq\isactrlesub \ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F\isactrlbsub Seq\isactrlesub \ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent
+ The functions \isa{simp\isactrlbsub Alt\isactrlesub } and \isa{simp\isactrlbsub Seq\isactrlesub } encode the simplification rules
+ in \eqref{Simpl} and compose the rectification functions (simplifications can occur
+ deep inside the regular expression). The main simplification function is then
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{simp\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{simp\isactrlbsub Alt\isactrlesub \ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{simp\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} & $\dn$ & \isa{simp\isactrlbsub Seq\isactrlesub \ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}}\\
+ \isa{simp\ r} & $\dn$ & \isa{{\isacharparenleft}{\kern0pt}r{\isacharcomma}{\kern0pt}\ id{\isacharparenright}{\kern0pt}}\\
+ \end{tabular}
+ \end{center}
+
+ \noindent where \isa{id} stands for the identity function. The
+ function \isa{simp} returns a simplified regular expression and a corresponding
+ rectification function. Note that we do not simplify under stars: this
+ seems to slow down the algorithm, rather than speed it up. The optimised
+ lexer is then given by the clauses:
+
+ \begin{center}
+ \begin{tabular}{lcl}
+ \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}\ r\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} & $\dn$ & \isa{\textrm{if}\ nullable\ r\ \textrm{then}\ Some\ {\isacharparenleft}{\kern0pt}mkeps\ r{\isacharparenright}{\kern0pt}\ \textrm{else}\ None}\\
+ \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}\ r\ {\isacharparenleft}{\kern0pt}c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s{\isacharparenright}{\kern0pt}} & $\dn$ &
+ \isa{let\ {\isacharparenleft}{\kern0pt}r\isactrlsub s{\isacharcomma}{\kern0pt}\ f\isactrlsub r{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ simp\ {\isacharparenleft}{\kern0pt}r}$\backslash$\isa{c{\isacharparenright}{\kern0pt}\ in}\\
+ & & \isa{case} \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}\ r\isactrlsub s\ s} \isa{of}\\
+ & & \phantom{$|$} \isa{None} \isa{{\isasymRightarrow}} \isa{None}\\
+ & & $|$ \isa{Some\ v} \isa{{\isasymRightarrow}} \isa{Some\ {\isacharparenleft}{\kern0pt}inj\ r\ c\ {\isacharparenleft}{\kern0pt}f\isactrlsub r\ v{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}}
+ \end{tabular}
+ \end{center}
+
+ \noindent
+ In the second clause we first calculate the derivative \isa{r{\isacharbackslash}{\kern0pt}c}
+ and then simpli
+
+text \isa{\ \ Incremental\ calculation\ of\ the\ value{\isachardot}{\kern0pt}\ To\ simplify\ the\ proof\ we\ first\ define\ the\ function\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}const\ flex{\isacharbraceright}{\kern0pt}\ which\ calculates\ the\ {\isacharbackquote}{\kern0pt}{\isacharbackquote}{\kern0pt}iterated{\isacharprime}{\kern0pt}{\isacharprime}{\kern0pt}\ injection\ function{\isachardot}{\kern0pt}\ With\ this\ we\ can\ rewrite\ the\ lexer\ as\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ lexer{\isacharunderscore}{\kern0pt}flex{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ {\isachardoublequote}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ {\isachardoublequote}{\kern0pt}v\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}v\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{7}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ code{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{7}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ areg{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}{\isacharequal}{\kern0pt}{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}AZERO{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}mid{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}AONE\ bs{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}mid{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}ACHAR\ bs\ c{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}mid{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}AALT\ bs\ r{\isadigit{1}}\ r{\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}mid{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}ASEQ\ bs\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}mid{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}ASTAR\ bs\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ intern{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ erase{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ Some\ simple\ facts\ about\ erase\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}lemma{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}mbox{\isacharbraceleft}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ erase{\isacharunderscore}{\kern0pt}bder{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ erase{\isacharunderscore}{\kern0pt}intern{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}lemma{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bnullable{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}medskip{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ \ {\isacharpercent}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharpercent}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ {\isacharpercent}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharpercent}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ \ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{5}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bder{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{6}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{3}}{\isacharparenright}{\kern0pt}{\isacharbrackleft}{\kern0pt}of\ bs\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}\ {\isachardoublequote}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ bmkeps{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{4}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}medskip{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ \ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharbrackleft}{\kern0pt}mode{\isacharequal}{\kern0pt}IfThen{\isacharbrackright}{\kern0pt}\ bder{\isacharunderscore}{\kern0pt}retrieve{\isacharbraceright}{\kern0pt}\ \ By\ induction\ on\ {\isasymopen}r{\isasymclose}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}theorem{\isacharbraceright}{\kern0pt}{\isacharbrackleft}{\kern0pt}Main\ Lemma{\isacharbrackright}{\kern0pt}{\isacharbackslash}{\kern0pt}mbox{\isacharbraceleft}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharbrackleft}{\kern0pt}mode{\isacharequal}{\kern0pt}IfThen{\isacharbrackright}{\kern0pt}\ MAIN{\isacharunderscore}{\kern0pt}decode{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}theorem{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}noindent\ Definition\ of\ the\ bitcoded\ lexer\ \ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ blexer{\isacharunderscore}{\kern0pt}def{\isacharbraceright}{\kern0pt}\ \ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}theorem{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ blexer{\isacharunderscore}{\kern0pt}correctness{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}theorem{\isacharbraceright}{\kern0pt}\ \ }
+
+section \isa{Optimisations}
+
+text \isa{\ \ Derivatives\ as\ calculated\ by\ {\isacharbackslash}{\kern0pt}Brz{\isacharprime}{\kern0pt}s\ method\ are\ usually\ more\ complex\ regular\ expressions\ than\ the\ initial\ one{\isacharsemicolon}{\kern0pt}\ the\ result\ is\ that\ the\ derivative{\isacharminus}{\kern0pt}based\ matching\ and\ lexing\ algorithms\ are\ often\ abysmally\ slow{\isachardot}{\kern0pt}\ However{\isacharcomma}{\kern0pt}\ various\ optimisations\ are\ possible{\isacharcomma}{\kern0pt}\ such\ as\ the\ simplifications\ of\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}ALT\ ZERO\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharcomma}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}ALT\ r\ ZERO{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharcomma}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}SEQ\ ONE\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}SEQ\ r\ ONE{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ to\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ r{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ These\ simplifications\ can\ speed\ up\ the\ algorithms\ considerably{\isacharcomma}{\kern0pt}\ as\ noted\ in\ {\isacharbackslash}{\kern0pt}cite{\isacharbraceleft}{\kern0pt}Sulzmann{\isadigit{2}}{\isadigit{0}}{\isadigit{1}}{\isadigit{4}}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ One\ of\ the\ advantages\ of\ having\ a\ simple\ specification\ and\ correctness\ proof\ is\ that\ the\ latter\ can\ be\ refined\ to\ prove\ the\ correctness\ of\ such\ simplification\ steps{\isachardot}{\kern0pt}\ While\ the\ simplification\ of\ regular\ expressions\ according\ to\ rules\ like\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}equation{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}label{\isacharbraceleft}{\kern0pt}Simpl{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}array{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcllcllcllcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}ALT\ ZERO\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}{\isasymRightarrow}{\isasymclose}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ r{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}hspace{\isacharbraceleft}{\kern0pt}{\isadigit{8}}mm{\isacharbraceright}{\kern0pt}{\isacharpercent}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}ALT\ r\ ZERO{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}{\isasymRightarrow}{\isasymclose}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ r{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}hspace{\isacharbraceleft}{\kern0pt}{\isadigit{8}}mm{\isacharbraceright}{\kern0pt}{\isacharpercent}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}SEQ\ ONE\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ \ {\isacharampersand}{\kern0pt}\ {\isasymopen}{\isasymRightarrow}{\isasymclose}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ r{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}hspace{\isacharbraceleft}{\kern0pt}{\isadigit{8}}mm{\isacharbraceright}{\kern0pt}{\isacharpercent}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}SEQ\ r\ ONE{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ \ {\isacharampersand}{\kern0pt}\ {\isasymopen}{\isasymRightarrow}{\isasymclose}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ r{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}array{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}equation{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}noindent\ is\ well\ understood{\isacharcomma}{\kern0pt}\ there\ is\ an\ obstacle\ with\ the\ POSIX\ value\ calculation\ algorithm\ by\ Sulzmann\ and\ Lu{\isacharcolon}{\kern0pt}\ if\ we\ build\ a\ derivative\ regular\ expression\ and\ then\ simplify\ it{\isacharcomma}{\kern0pt}\ we\ will\ calculate\ a\ POSIX\ value\ for\ this\ simplified\ derivative\ regular\ expression{\isacharcomma}{\kern0pt}\ {\isacharbackslash}{\kern0pt}emph{\isacharbraceleft}{\kern0pt}not{\isacharbraceright}{\kern0pt}\ for\ the\ original\ {\isacharparenleft}{\kern0pt}unsimplified{\isacharparenright}{\kern0pt}\ derivative\ regular\ expression{\isachardot}{\kern0pt}\ Sulzmann\ and\ Lu\ {\isacharbackslash}{\kern0pt}cite{\isacharbraceleft}{\kern0pt}Sulzmann{\isadigit{2}}{\isadigit{0}}{\isadigit{1}}{\isadigit{4}}{\isacharbraceright}{\kern0pt}\ overcome\ this\ obstacle\ by\ not\ just\ calculating\ a\ simplified\ regular\ expression{\isacharcomma}{\kern0pt}\ but\ also\ calculating\ a\ {\isacharbackslash}{\kern0pt}emph{\isacharbraceleft}{\kern0pt}rectification\ function{\isacharbraceright}{\kern0pt}\ that\ {\isacharbackquote}{\kern0pt}{\isacharbackquote}{\kern0pt}repairs{\isacharprime}{\kern0pt}{\isacharprime}{\kern0pt}\ the\ incorrect\ value{\isachardot}{\kern0pt}\ \ The\ rectification\ functions\ can\ be\ {\isacharparenleft}{\kern0pt}slightly\ clumsily{\isacharparenright}{\kern0pt}\ implemented\ \ in\ Isabelle{\isacharslash}{\kern0pt}HOL\ as\ follows\ using\ some\ auxiliary\ functions{\isacharcolon}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}RIGHT{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}Right\ {\isacharparenleft}{\kern0pt}f\ v{\isacharparenright}{\kern0pt}{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}LEFT{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}Left\ {\isacharparenleft}{\kern0pt}f\ v{\isacharparenright}{\kern0pt}{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ \ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}ALT{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}Right\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ v{\isacharparenright}{\kern0pt}{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}ALT{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}Left\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ v{\isacharparenright}{\kern0pt}{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ \ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}SEQ{\isadigit{1}}{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}Seq\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ {\isacharparenleft}{\kern0pt}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ v{\isacharparenright}{\kern0pt}{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}SEQ{\isadigit{2}}{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}Seq\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ v{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ {\isacharparenleft}{\kern0pt}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}SEQ{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}Seq\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{1}}\ v\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}f\isactrlsub {\isadigit{2}}\ v\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isasymclose}{\isacharbackslash}{\kern0pt}medskip{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharpercent}{\kern0pt}{\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharpercent}{\kern0pt}\ {\isacharpercent}{\kern0pt}{\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}ALT\ {\isacharparenleft}{\kern0pt}ZERO{\isacharcomma}{\kern0pt}\ DUMMY{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}RIGHT\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}ALT\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}ZERO{\isacharcomma}{\kern0pt}\ DUMMY{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}LEFT\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}ALT\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharparenleft}{\kern0pt}ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}ALT\ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}SEQ\ {\isacharparenleft}{\kern0pt}ONE{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}SEQ{\isadigit{1}}\ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}SEQ\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}ONE{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}SEQ{\isadigit{2}}\ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}SEQ\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharparenleft}{\kern0pt}SEQ\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isacharcomma}{\kern0pt}\ F{\isacharunderscore}{\kern0pt}SEQ\ f\isactrlsub {\isadigit{1}}\ f\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}noindent\ The\ functions\ {\isasymopen}simp\isactrlbsub Alt\isactrlesub {\isasymclose}\ and\ {\isasymopen}simp\isactrlbsub Seq\isactrlesub {\isasymclose}\ encode\ the\ simplification\ rules\ in\ {\isacharbackslash}{\kern0pt}eqref{\isacharbraceleft}{\kern0pt}Simpl{\isacharbraceright}{\kern0pt}\ and\ compose\ the\ rectification\ functions\ {\isacharparenleft}{\kern0pt}simplifications\ can\ occur\ deep\ inside\ the\ regular\ expression{\isacharparenright}{\kern0pt}{\isachardot}{\kern0pt}\ The\ main\ simplification\ function\ is\ then\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}ALT\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}SEQ\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp{\isacharunderscore}{\kern0pt}SEQ\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}simp\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharparenleft}{\kern0pt}r{\isacharcomma}{\kern0pt}\ id{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}noindent\ where\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}id{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ stands\ for\ the\ identity\ function{\isachardot}{\kern0pt}\ The\ function\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}const\ simp{\isacharbraceright}{\kern0pt}\ returns\ a\ simplified\ regular\ expression\ and\ a\ corresponding\ rectification\ function{\isachardot}{\kern0pt}\ Note\ that\ we\ do\ not\ simplify\ under\ stars{\isacharcolon}{\kern0pt}\ this\ seems\ to\ slow\ down\ the\ algorithm{\isacharcomma}{\kern0pt}\ rather\ than\ speed\ it\ up{\isachardot}{\kern0pt}\ The\ optimised\ lexer\ is\ then\ given\ by\ the\ clauses{\isacharcolon}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}lcl{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ slexer{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}rhs{\isacharparenright}{\kern0pt}\ slexer{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ {\isacharparenleft}{\kern0pt}lhs{\isacharparenright}{\kern0pt}\ slexer{\isachardot}{\kern0pt}simps{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}dn{\isachardollar}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}let\ {\isacharparenleft}{\kern0pt}r\isactrlsub s{\isacharcomma}{\kern0pt}\ f\isactrlsub r{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ simp\ {\isacharparenleft}{\kern0pt}r\ {\isasymclose}{\isachardollar}{\kern0pt}{\isacharbackslash}{\kern0pt}backslash{\isachardollar}{\kern0pt}{\isasymopen}\ c{\isacharparenright}{\kern0pt}\ in{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isasymopen}case{\isasymclose}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}slexer\ r\isactrlsub s\ s{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isasymopen}of{\isasymclose}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharbackslash}{\kern0pt}phantom{\isacharbraceleft}{\kern0pt}{\isachardollar}{\kern0pt}{\isacharbar}{\kern0pt}{\isachardollar}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}None{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ \ {\isasymopen}{\isasymRightarrow}{\isasymclose}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ None{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isachardollar}{\kern0pt}{\isacharbar}{\kern0pt}{\isachardollar}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}Some\ v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ {\isasymopen}{\isasymRightarrow}{\isasymclose}\ {\isasymopen}Some\ {\isacharparenleft}{\kern0pt}inj\ r\ c\ {\isacharparenleft}{\kern0pt}f\isactrlsub r\ v{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isasymclose}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}center{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}noindent\ In\ the\ second\ clause\ we\ first\ calculate\ the\ derivative\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}der\ c\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ then\ simplify\ the\ result{\isachardot}{\kern0pt}\ This\ gives\ us\ a\ simplified\ derivative\ {\isasymopen}r\isactrlsub s{\isasymclose}\ and\ a\ rectification\ function\ {\isasymopen}f\isactrlsub r{\isasymclose}{\isachardot}{\kern0pt}\ The\ lexer\ is\ then\ recursively\ called\ with\ the\ simplified\ derivative{\isacharcomma}{\kern0pt}\ but\ before\ we\ inject\ the\ character\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ c{\isacharbraceright}{\kern0pt}\ into\ the\ value\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ v{\isacharbraceright}{\kern0pt}{\isacharcomma}{\kern0pt}\ we\ need\ to\ rectify\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ v{\isacharbraceright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}that\ is\ construct\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}f\isactrlsub r\ v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isachardot}{\kern0pt}\ Before\ we\ can\ establish\ the\ correctness\ of\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}slexer{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharcomma}{\kern0pt}\ we\ need\ to\ show\ that\ simplification\ preserves\ the\ language\ and\ simplification\ preserves\ our\ POSIX\ relation\ once\ the\ value\ is\ rectified\ {\isacharparenleft}{\kern0pt}recall\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}const\ {\isachardoublequote}{\kern0pt}simp{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ generates\ a\ {\isacharparenleft}{\kern0pt}regular\ expression{\isacharcomma}{\kern0pt}\ rectification\ function{\isacharparenright}{\kern0pt}\ pair{\isacharparenright}{\kern0pt}{\isacharcolon}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}lemma{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}mbox{\isacharbraceleft}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}smallskip{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}label{\isacharbraceleft}{\kern0pt}slexeraux{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}{\isacharbraceleft}{\kern0pt}ll{\isacharbraceright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ L{\isacharunderscore}{\kern0pt}fst{\isacharunderscore}{\kern0pt}simp{\isacharbrackleft}{\kern0pt}symmetric{\isacharbrackright}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharbackslash}{\kern0pt}{\isacharbackslash}{\kern0pt}\ {\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isacharampersand}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm{\isacharbrackleft}{\kern0pt}mode{\isacharequal}{\kern0pt}IfThen{\isacharbrackright}{\kern0pt}\ Posix{\isacharunderscore}{\kern0pt}simp{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}tabular{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}lemma{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}proof{\isacharbraceright}{\kern0pt}\ Both\ are\ by\ induction\ on\ {\isasymopen}r{\isasymclose}{\isachardot}{\kern0pt}\ There\ is\ no\ interesting\ case\ for\ the\ first\ statement{\isachardot}{\kern0pt}\ For\ the\ second\ statement{\isacharcomma}{\kern0pt}\ of\ interest\ are\ the\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}r\ {\isacharequal}{\kern0pt}\ ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}r\ {\isacharequal}{\kern0pt}\ SEQ\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ cases{\isachardot}{\kern0pt}\ In\ each\ case\ we\ have\ to\ analyse\ four\ subcases\ whether\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ equals\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}const\ ZERO{\isacharbraceright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}respectively\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}const\ ONE{\isacharbraceright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isachardot}{\kern0pt}\ For\ example\ for\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}r\ {\isacharequal}{\kern0pt}\ ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharcomma}{\kern0pt}\ consider\ the\ subcase\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ ZERO{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymnoteq}\ ZERO{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ By\ assumption\ we\ know\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ fst\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ From\ this\ we\ can\ infer\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ by\ IH\ also\ {\isacharparenleft}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ r\isactrlsub {\isadigit{2}}\ {\isasymrightarrow}\ {\isacharparenleft}{\kern0pt}snd\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ v{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ Given\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ ZERO{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ we\ know\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}L\ {\isacharparenleft}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isacharbraceleft}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ By\ the\ first\ statement\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}L\ r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ is\ the\ empty\ set{\isacharcomma}{\kern0pt}\ meaning\ {\isacharparenleft}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymnotin}\ L\ r\isactrlsub {\isadigit{1}}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ Taking\ {\isacharparenleft}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharparenright}{\kern0pt}\ and\ {\isacharparenleft}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharparenright}{\kern0pt}\ together\ gives\ by\ the\ {\isacharbackslash}{\kern0pt}mbox{\isacharbraceleft}{\kern0pt}{\isasymopen}P{\isacharplus}{\kern0pt}R{\isasymclose}{\isacharbraceright}{\kern0pt}{\isacharminus}{\kern0pt}rule\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}\ {\isasymrightarrow}\ Right\ {\isacharparenleft}{\kern0pt}snd\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ v{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ In\ turn\ this\ gives\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}\ {\isasymrightarrow}\ snd\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}ALT\ r\isactrlsub {\isadigit{1}}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ as\ we\ need\ to\ show{\isachardot}{\kern0pt}\ The\ other\ cases\ are\ similar{\isachardot}{\kern0pt}{\isacharbackslash}{\kern0pt}qed\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}proof{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}noindent\ We\ can\ now\ prove\ relatively\ straightforwardly\ that\ the\ optimised\ lexer\ produces\ the\ expected\ result{\isacharcolon}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}theorem{\isacharbraceright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}thm\ slexer{\isacharunderscore}{\kern0pt}correctness{\isacharbraceright}{\kern0pt}\ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}theorem{\isacharbraceright}{\kern0pt}\ \ {\isacharbackslash}{\kern0pt}begin{\isacharbraceleft}{\kern0pt}proof{\isacharbraceright}{\kern0pt}\ By\ induction\ on\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ s{\isacharbraceright}{\kern0pt}\ generalising\ over\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ r{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ The\ case\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ is\ trivial{\isachardot}{\kern0pt}\ For\ the\ cons{\isacharminus}{\kern0pt}case\ suppose\ the\ string\ is\ of\ the\ form\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}c\ {\isacharhash}{\kern0pt}\ s{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ By\ induction\ hypothesis\ we\ know\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}slexer\ r\ s\ {\isacharequal}{\kern0pt}\ lexer\ r\ s{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ holds\ for\ all\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ r{\isacharbraceright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}in\ particular\ for\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ being\ the\ derivative\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}der\ c\ r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isachardot}{\kern0pt}\ Let\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}r\isactrlsub s{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ be\ the\ simplified\ derivative\ regular\ expression{\isacharcomma}{\kern0pt}\ that\ is\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}der\ c\ r{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharcomma}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}f\isactrlsub r{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ be\ the\ rectification\ function{\isacharcomma}{\kern0pt}\ that\ is\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}snd\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}der\ c\ r{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ \ We\ distinguish\ the\ cases\ whether\ {\isacharparenleft}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ L\ {\isacharparenleft}{\kern0pt}der\ c\ r{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ or\ not{\isachardot}{\kern0pt}\ In\ the\ first\ case\ we\ have\ by\ Theorem{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}lexercorrect{\isacharbraceright}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}\ a\ value\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ so\ that\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}lexer\ {\isacharparenleft}{\kern0pt}der\ c\ r{\isacharparenright}{\kern0pt}\ s\ {\isacharequal}{\kern0pt}\ Some\ v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ der\ c\ r\ {\isasymrightarrow}\ v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ hold{\isachardot}{\kern0pt}\ By\ Lemma{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}slexeraux{\isacharbraceright}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}\ we\ can\ also\ infer\ from{\isachartilde}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isacharasterisk}{\kern0pt}{\isacharparenright}{\kern0pt}\ that\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ L\ r\isactrlsub s{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ holds{\isachardot}{\kern0pt}\ \ Hence\ we\ know\ by\ Theorem{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}lexercorrect{\isacharbraceright}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}\ that\ there\ exists\ a\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}v{\isacharprime}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ with\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}lexer\ r\isactrlsub s\ s\ {\isacharequal}{\kern0pt}\ Some\ v{\isacharprime}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ r\isactrlsub s\ {\isasymrightarrow}\ v{\isacharprime}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ From\ the\ latter\ we\ know\ by\ Lemma{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}slexeraux{\isacharbraceright}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isadigit{2}}{\isacharparenright}{\kern0pt}\ that\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymin}\ der\ c\ r\ {\isasymrightarrow}\ {\isacharparenleft}{\kern0pt}f\isactrlsub r\ v{\isacharprime}{\kern0pt}{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ holds{\isachardot}{\kern0pt}\ By\ the\ uniqueness\ of\ the\ POSIX\ relation\ {\isacharparenleft}{\kern0pt}Theorem{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}posixdeterm{\isacharbraceright}{\kern0pt}{\isacharparenright}{\kern0pt}\ we\ can\ infer\ that\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ v{\isacharbraceright}{\kern0pt}\ is\ equal\ to\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}f\isactrlsub r\ v{\isacharprime}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isacharminus}{\kern0pt}{\isacharminus}{\kern0pt}{\isacharminus}{\kern0pt}that\ is\ the\ rectification\ function\ applied\ to\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}v{\isacharprime}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ produces\ the\ original\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}v{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ \ Now\ the\ case\ follows\ by\ the\ definitions\ of\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}const\ lexer{\isacharbraceright}{\kern0pt}\ and\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}const\ slexer{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ \ In\ the\ second\ case\ where\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymnotin}\ L\ {\isacharparenleft}{\kern0pt}der\ c\ r{\isacharparenright}{\kern0pt}{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ we\ have\ that\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}lexer\ {\isacharparenleft}{\kern0pt}der\ c\ r{\isacharparenright}{\kern0pt}\ s\ {\isacharequal}{\kern0pt}\ None{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ by\ Theorem{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}lexercorrect{\isacharbraceright}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}{\isachardot}{\kern0pt}\ \ We\ also\ know\ by\ Lemma{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}slexeraux{\isacharbraceright}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}\ that\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}s\ {\isasymnotin}\ L\ r\isactrlsub s{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ Hence\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}lexer\ r\isactrlsub s\ s\ {\isacharequal}{\kern0pt}\ None{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}\ by\ Theorem{\isachartilde}{\kern0pt}{\isacharbackslash}{\kern0pt}ref{\isacharbraceleft}{\kern0pt}lexercorrect{\isacharbraceright}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isadigit{1}}{\isacharparenright}{\kern0pt}\ and\ by\ IH\ then\ also\ {\isacharat}{\kern0pt}{\isacharbraceleft}{\kern0pt}term\ {\isachardoublequote}{\kern0pt}slexer\ r\isactrlsub s\ s\ {\isacharequal}{\kern0pt}\ None{\isachardoublequote}{\kern0pt}{\isacharbraceright}{\kern0pt}{\isachardot}{\kern0pt}\ With\ this\ we\ can\ conclude\ in\ this\ case\ too{\isachardot}{\kern0pt}{\isacharbackslash}{\kern0pt}qed\ \ {\isacharbackslash}{\kern0pt}end{\isacharbraceleft}{\kern0pt}proof{\isacharbraceright}{\kern0pt}\ \ }
+fy the result. This gives us a simplified derivative
+ \isa{r\isactrlsub s} and a rectification function \isa{f\isactrlsub r}. The lexer
+ is then recursively called with the simplified derivative, but before
+ we inject the character \isa{c} into the value \isa{v}, we need to rectify
+ \isa{v} (that is construct \isa{f\isactrlsub r\ v}). Before we can establish the correctness
+ of \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}}, we need to show that simplification preserves the language
+ and simplification preserves our POSIX relation once the value is rectified
+ (recall \isa{simp} generates a (regular expression, rectification function) pair):
+
+ \begin{lemma}\mbox{}\smallskip\\\label{slexeraux}
+ \begin{tabular}{ll}
+ (1) & \isa{L{\isacharparenleft}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ L{\isacharparenleft}{\kern0pt}r{\isacharparenright}{\kern0pt}}\\
+ (2) & \isa{{\normalsize{}If\,}\ {\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ fst\ {\isacharparenleft}{\kern0pt}simp\ r{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v\ {\normalsize \,then\,}\ {\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ snd\ {\isacharparenleft}{\kern0pt}simp\ r{\isacharparenright}{\kern0pt}\ v{\isachardot}{\kern0pt}}
+ \end{tabular}
+ \end{lemma}
+
+ \begin{proof} Both are by induction on \isa{r}. There is no
+ interesting case for the first statement. For the second statement,
+ of interest are the \isa{r\ {\isacharequal}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}} and \isa{r\ {\isacharequal}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isasymcdot}\ r\isactrlsub {\isadigit{2}}} cases. In each case we have to analyse four subcases whether
+ \isa{fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}} and \isa{fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}} equals \isa{\isactrlbold {\isadigit{0}}} (respectively \isa{\isactrlbold {\isadigit{1}}}). For example for \isa{r\ {\isacharequal}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}}, consider the subcase \isa{fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ \isactrlbold {\isadigit{0}}} and
+ \isa{fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymnoteq}\ \isactrlbold {\isadigit{0}}}. By assumption we know \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ fst\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}. From this we can infer \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v}
+ and by IH also (*) \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ snd\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ v}. Given \isa{fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ \isactrlbold {\isadigit{0}}}
+ we know \isa{L{\isacharparenleft}{\kern0pt}fst\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharequal}{\kern0pt}\ {\isasymemptyset}}. By the first statement
+ \isa{L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}} is the empty set, meaning (**) \isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}{\isacharparenright}{\kern0pt}}.
+ Taking (*) and (**) together gives by the \mbox{\isa{P{\isacharplus}{\kern0pt}R}}-rule
+ \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ Right\ {\isacharparenleft}{\kern0pt}snd\ {\isacharparenleft}{\kern0pt}simp\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ v{\isacharparenright}{\kern0pt}}. In turn this
+ gives \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ snd\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\ {\isacharplus}{\kern0pt}\ r\isactrlsub {\isadigit{2}}{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ v} as we need to show.
+ The other cases are similar.\qed
+ \end{proof}
+
+ \noindent We can now prove relatively straightforwardly that the
+ optimised lexer produces the expected result:
+
+ \begin{theorem}
+ \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}\ r\ s\ {\isacharequal}{\kern0pt}\ lexer\ r\ s}
+ \end{theorem}
+
+ \begin{proof} By induction on \isa{s} generalising over \isa{r}. The case \isa{{\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}} is trivial. For the cons-case suppose the
+ string is of the form \isa{c\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}s}. By induction hypothesis we
+ know \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}\ r\ s\ {\isacharequal}{\kern0pt}\ lexer\ r\ s} holds for all \isa{r} (in
+ particular for \isa{r} being the derivative \isa{r{\isacharbackslash}{\kern0pt}c}). Let \isa{r\isactrlsub s} be the simplified derivative regular expression, that is \isa{fst\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}}, and \isa{f\isactrlsub r} be the rectification
+ function, that is \isa{snd\ {\isacharparenleft}{\kern0pt}simp\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}}. We distinguish the cases
+ whether (*) \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}} or not. In the first case we
+ have by Theorem~\ref{lexercorrect}(2) a value \isa{v} so that \isa{lexer\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ s\ {\isacharequal}{\kern0pt}\ Some\ v} and \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v} hold.
+ By Lemma~\ref{slexeraux}(1) we can also infer from~(*) that \isa{s\ {\isasymin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub s{\isacharparenright}{\kern0pt}} holds. Hence we know by Theorem~\ref{lexercorrect}(2) that
+ there exists a \isa{v{\isacharprime}{\kern0pt}} with \isa{lexer\ r\isactrlsub s\ s\ {\isacharequal}{\kern0pt}\ Some\ v{\isacharprime}{\kern0pt}} and
+ \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r\isactrlsub s{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ v{\isacharprime}{\kern0pt}}. From the latter we know by
+ Lemma~\ref{slexeraux}(2) that \isa{{\isacharparenleft}{\kern0pt}s{\isacharcomma}{\kern0pt}\ r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymrightarrow}\ f\isactrlsub r\ v{\isacharprime}{\kern0pt}} holds.
+ By the uniqueness of the POSIX relation (Theorem~\ref{posixdeterm}) we
+ can infer that \isa{v} is equal to \isa{f\isactrlsub r\ v{\isacharprime}{\kern0pt}}---that is the
+ rectification function applied to \isa{v{\isacharprime}{\kern0pt}}
+ produces the original \isa{v}. Now the case follows by the
+ definitions of \isa{lexer} and \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}}.
+
+ In the second case where \isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}} we have that
+ \isa{lexer\ {\isacharparenleft}{\kern0pt}r{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ s\ {\isacharequal}{\kern0pt}\ None} by Theorem~\ref{lexercorrect}(1). We
+ also know by Lemma~\ref{slexeraux}(1) that \isa{s\ {\isasymnotin}\ L{\isacharparenleft}{\kern0pt}r\isactrlsub s{\isacharparenright}{\kern0pt}}. Hence
+ \isa{lexer\ r\isactrlsub s\ s\ {\isacharequal}{\kern0pt}\ None} by Theorem~\ref{lexercorrect}(1) and
+ by IH then also \isa{lexer\isactrlsup {\isacharplus}{\kern0pt}\ r\isactrlsub s\ s\ {\isacharequal}{\kern0pt}\ None}. With this we can
+ conclude in this case too.\qed
+
+ \end{proof}%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\isatagdocument
+%
+\isamarkupsection{HERE%
+}
+\isamarkuptrue%
+%
+\endisatagdocument
+{\isafolddocument}%
+%
+\isadelimdocument
+%
+\endisadelimdocument
+%
+\begin{isamarkuptext}%
+\begin{lemma}
+ \isa{{\normalsize{}If\,}\ v\ {\isacharcolon}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c\ {\normalsize \,then\,}\ retrieve\ {\isacharparenleft}{\kern0pt}r\mbox{$\bbslash$}c{\isacharparenright}{\kern0pt}\ v\ {\isacharequal}{\kern0pt}\ retrieve\ r\ {\isacharparenleft}{\kern0pt}inj\ {\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}\ c\ v{\isacharparenright}{\kern0pt}{\isachardot}{\kern0pt}}
+ \end{lemma}
+
+ \begin{proof}
+ By induction on the definition of \isa{r\mbox{$^\downarrow$}}. The cases for rule 1) and 2) are
+ straightforward as \isa{\isactrlbold {\isadigit{0}}{\isacharbackslash}{\kern0pt}c} and \isa{\isactrlbold {\isadigit{1}}{\isacharbackslash}{\kern0pt}c} are both equal to
+ \isa{\isactrlbold {\isadigit{0}}}. This means \isa{v\ {\isacharcolon}{\kern0pt}\ \isactrlbold {\isadigit{0}}} cannot hold. Similarly in case of rule 3)
+ where \isa{r} is of the form \isa{ACHAR\ d} with \isa{c\ {\isacharequal}{\kern0pt}\ d}. Then by assumption
+ we know \isa{v\ {\isacharcolon}{\kern0pt}\ \isactrlbold {\isadigit{1}}}, which implies \isa{v\ {\isacharequal}{\kern0pt}\ Empty}. The equation follows by
+ simplification of left- and right-hand side. In case \isa{c\ {\isasymnoteq}\ d} we have again
+ \isa{v\ {\isacharcolon}{\kern0pt}\ \isactrlbold {\isadigit{0}}}, which cannot hold.
+
+ For rule 4a) we have again \isa{v\ {\isacharcolon}{\kern0pt}\ \isactrlbold {\isadigit{0}}}. The property holds by IH for rule 4b).
+ The induction hypothesis is
+ \[
+ \isa{retrieve\ {\isacharparenleft}{\kern0pt}r\mbox{$\bbslash$}c{\isacharparenright}{\kern0pt}\ v\ {\isacharequal}{\kern0pt}\ retrieve\ r\ {\isacharparenleft}{\kern0pt}inj\ {\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}\ c\ v{\isacharparenright}{\kern0pt}}
+ \]
+ which is what left- and right-hand side simplify to. The slightly more interesting case
+ is for 4c). By assumption we have
+ \isa{v\ {\isacharcolon}{\kern0pt}\ {\isacharparenleft}{\kern0pt}{\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isacharplus}{\kern0pt}\ {\isacharparenleft}{\kern0pt}{\isacharparenleft}{\kern0pt}{\isacharparenleft}{\kern0pt}AALTs\ bs\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}rs{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}}. This means we
+ have either (*) \isa{v{\isadigit{1}}\ {\isacharcolon}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{1}}\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c} with \isa{v\ {\isacharequal}{\kern0pt}\ Left\ v{\isadigit{1}}} or
+ (**) \isa{v{\isadigit{2}}\ {\isacharcolon}{\kern0pt}\ {\isacharparenleft}{\kern0pt}{\isacharparenleft}{\kern0pt}AALTs\ bs\ {\isacharparenleft}{\kern0pt}r\isactrlsub {\isadigit{2}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}rs{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c} with \isa{v\ {\isacharequal}{\kern0pt}\ Right\ v{\isadigit{2}}}.
+ The former case is straightforward by simplification. The second case is \ldots TBD.
+
+ Rule 5) TBD.
+
+ Finally for rule 6) the reasoning is as follows: By assumption we have
+ \isa{v\ {\isacharcolon}{\kern0pt}\ {\isacharparenleft}{\kern0pt}{\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c{\isacharparenright}{\kern0pt}\ {\isasymcdot}\ {\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}\isactrlsup {\isasymstar}}. This means we also have
+ \isa{v\ {\isacharequal}{\kern0pt}\ Seq\ v{\isadigit{1}}\ v{\isadigit{2}}}, \isa{v{\isadigit{1}}\ {\isacharcolon}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}{\isacharbackslash}{\kern0pt}c} and \isa{v{\isadigit{2}}\ {\isacharequal}{\kern0pt}\ Stars\ vs}.
+ We want to prove
+ \begin{align}
+ & \isa{retrieve\ {\isacharparenleft}{\kern0pt}ASEQ\ bs\ {\isacharparenleft}{\kern0pt}fuse\ {\isacharbrackleft}{\kern0pt}Z{\isacharbrackright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\mbox{$\bbslash$}c{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}ASTAR\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ r{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ v}\\
+ &= \isa{retrieve\ {\isacharparenleft}{\kern0pt}ASTAR\ bs\ r{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}inj\ {\isacharparenleft}{\kern0pt}{\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}\isactrlsup {\isasymstar}{\isacharparenright}{\kern0pt}\ c\ v{\isacharparenright}{\kern0pt}}
+ \end{align}
+ The right-hand side \isa{inj}-expression is equal to
+ \isa{Stars\ {\isacharparenleft}{\kern0pt}inj\ {\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}\ c\ v{\isadigit{1}}\mbox{$\,$}{\isacharcolon}{\kern0pt}{\isacharcolon}{\kern0pt}\mbox{$\,$}vs{\isacharparenright}{\kern0pt}}, which means the \isa{retrieve}-expression
+ simplifies to
+ \[
+ \isa{bs\ {\isacharat}{\kern0pt}\ {\isacharbrackleft}{\kern0pt}Z{\isacharbrackright}{\kern0pt}\ {\isacharat}{\kern0pt}\ retrieve\ r\ {\isacharparenleft}{\kern0pt}inj\ {\isacharparenleft}{\kern0pt}r\mbox{$^\downarrow$}{\isacharparenright}{\kern0pt}\ c\ v{\isadigit{1}}{\isacharparenright}{\kern0pt}\ {\isacharat}{\kern0pt}\ retrieve\ {\isacharparenleft}{\kern0pt}ASTAR\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}Stars\ vs{\isacharparenright}{\kern0pt}}
+ \]
+ The left-hand side (3) above simplifies to
+ \[
+ \isa{bs\ {\isacharat}{\kern0pt}\ retrieve\ {\isacharparenleft}{\kern0pt}fuse\ {\isacharbrackleft}{\kern0pt}Z{\isacharbrackright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}r\mbox{$\bbslash$}c{\isacharparenright}{\kern0pt}{\isacharparenright}{\kern0pt}\ v{\isadigit{1}}\ {\isacharat}{\kern0pt}\ retrieve\ {\isacharparenleft}{\kern0pt}ASTAR\ {\isacharbrackleft}{\kern0pt}{\isacharbrackright}{\kern0pt}\ r{\isacharparenright}{\kern0pt}\ {\isacharparenleft}{\kern0pt}Stars\ vs{\isacharparenright}{\kern0pt}}
+ \]
+ We can move out the \isa{fuse\ {\isacharbrackleft}{\kern0pt}Z{\isacharbrackright}{\kern0pt}} and then use the IH to show that left-hand side
+ and right-hand side are equal. This completes the proof.
+ \end{proof}
+
+
+
+ \bibliographystyle{plain}
+ \bibliography{root}%
+\end{isamarkuptext}\isamarkuptrue%
+%
+\isadelimtheory
+%
+\endisadelimtheory
+%
+\isatagtheory
+%
+\endisatagtheory
+{\isafoldtheory}%
+%
+\isadelimtheory
+\isanewline
+%
+\endisadelimtheory
+\isanewline
+\isanewline
+%
+\end{isabellebody}%
+\endinput
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