
diff -r c028b07a4fa8 -r 99161cd17c0f Journal/Paper.thy
--- a/Journal/Paper.thy	Wed Jul 10 13:28:24 2013 +0100
+++ b/Journal/Paper.thy	Wed Jul 10 17:47:30 2013 +0100
@@ -563,7 +563,7 @@
 
   Central to our proof will be the solution of equational systems
   involving equivalence classes of languages. For this we will use Arden's Lemma 
-  (see for example \cite[Page 100]{Sakarovitch09}),
+  (see for example \citet[Page 100]{Sakarovitch09}),
   which solves equations of the form @{term "X = A \<cdot> X \<union> B"} provided
   @{term "[] \<notin> A"}. However we will need the following `reversed' 
   version of Arden's Lemma (`reversed' in the sense of changing the order of @{term "A \<cdot> X"} to
@@ -659,16 +659,18 @@
   It is easy to see that @{term "\<approx>A"} is an equivalence relation, which
   partitions the set of all strings, @{text "UNIV"}, into a set of disjoint
   equivalence classes. To illustrate this quotient construction, let us give a simple 
-  example: consider the regular language containing just
-  the string @{text "[c]"}. The relation @{term "\<approx>({[c]})"} partitions @{text UNIV}
-  into three equivalence classes @{text "X\<^isub>1"}, @{text "X\<^isub>2"} and  @{text "X\<^isub>3"}
+  example: consider the regular language built up over the alphabet @{term "{a, b}"} and 
+  containing just the string two strings @{text "[a]"} and @{text "[a, b]"}. The 
+  relation @{term "\<approx>({[a], [a, b]})"} partitions @{text UNIV}
+  into four equivalence classes @{text "X\<^isub>1"}, @{text "X\<^isub>2"}, @{text "X\<^isub>3"} and @{text "X\<^isub>4"}
   as follows
   
   \begin{center}
   \begin{tabular}{l}
   @{text "X\<^isub>1 = {[]}"}\\
-  @{text "X\<^isub>2 = {[c]}"}\\
-  @{text "X\<^isub>3 = UNIV - {[], [c]}"}
+  @{text "X\<^isub>2 = {[a]}"}\\
+  @{text "X\<^isub>3 = {[a, b]}"}\\
+  @{text "X\<^isub>4 = UNIV - {[], [a], [a, b]}"}
   \end{tabular}
   \end{center}
 
@@ -689,8 +691,8 @@
   \end{equation}
 
   \noindent
-  In our running example, @{text "X\<^isub>2"} is the only 
-  equivalence class in @{term "finals {[c]}"}.
+  In our running example, @{text "X\<^isub>2"} and @{text "X\<^isub>3"} are the only 
+  equivalence classes in @{term "finals {[a], [a, b]}"}.
   It is straightforward to show that in general 
   %
   \begin{equation}\label{finalprops}
@@ -723,10 +725,16 @@
   %Note that we do not define formally an automaton here, 
   %we merely relate two sets
   %(with the help of a character). 
-  In our concrete example we have 
-  @{term "X\<^isub>1 \<Turnstile>c\<Rightarrow> X\<^isub>2"}, @{term "X\<^isub>1 \<Turnstile>d\<^isub>i\<Rightarrow> X\<^isub>3"} with @{text "d\<^isub>i"} being any 
-  other character than @{text c}, and @{term "X\<^isub>3 \<Turnstile>c\<^isub>j\<Rightarrow> X\<^isub>3"} for any 
-  character @{text "c\<^isub>j"}.
+  In our concrete example we have
+
+  \begin{center} 
+  \begin{tabular}{l}
+  @{term "X\<^isub>1 \<Turnstile>a\<Rightarrow> X\<^isub>2"},\; @{term "X\<^isub>1 \<Turnstile>b\<Rightarrow> X\<^isub>4"};\\ 
+  @{term "X\<^isub>2 \<Turnstile>b\<Rightarrow> X\<^isub>3"},\; @{term "X\<^isub>2 \<Turnstile>a\<Rightarrow> X\<^isub>4"};\\
+  @{term "X\<^isub>3 \<Turnstile>a\<Rightarrow> X\<^isub>4"},\; @{term "X\<^isub>3 \<Turnstile>b\<Rightarrow> X\<^isub>4"} and\\ 
+  @{term "X\<^isub>4 \<Turnstile>a\<Rightarrow> X\<^isub>4"},\; @{term "X\<^isub>4 \<Turnstile>b\<Rightarrow> X\<^isub>4"}.
+  \end{tabular}
+  \end{center}
   
   Next we construct an \emph{initial equational system} that
   contains an equation for each equivalence class. We first give
@@ -773,17 +781,14 @@
   \begin{equation}\label{exmpcs}
   \mbox{\begin{tabular}{l@ {\hspace{1mm}}c@ {\hspace{1mm}}l}
   @{term "X\<^isub>1"} & @{text "="} & @{text "\<lambda>(ONE)"}\\
-  @{term "X\<^isub>2"} & @{text "="} & @{text "(X\<^isub>1, ATOM c)"}\\
-  @{term "X\<^isub>3"} & @{text "="} & @{text "(X\<^isub>1, ATOM d\<^isub>1) + \<dots> + (X\<^isub>1, ATOM d\<^isub>n)"}\\
-               & & \mbox{}\hspace{10mm}@{text "+ (X\<^isub>3, ATOM c\<^isub>1) + \<dots> + (X\<^isub>3, ATOM c\<^isub>m)"}
+  @{term "X\<^isub>2"} & @{text "="} & @{text "(X\<^isub>1, ATOM a)"}\\
+  @{term "X\<^isub>3"} & @{text "="} & @{text "(X\<^isub>2, ATOM b)"}\\
+  @{term "X\<^isub>4"} & @{text "="} & @{text "(X\<^isub>1, ATOM b) + (X\<^isub>2, ATOM a) + (X\<^isub>3, ATOM a)"}\\
+          & & \mbox{}\hspace{0mm}@{text "+ (X\<^isub>3, ATOM b) + (X\<^isub>4, ATOM a) + (X\<^isub>4, ATOM b)"}\\
   \end{tabular}}
   \end{equation}
-  
+
   \noindent
-  where @{text "d\<^isub>1\<dots>d\<^isub>n"} is the sequence of all characters
-  but not containing @{text c}, and @{text "c\<^isub>1\<dots>c\<^isub>m"} is the sequence of all
-  characters.  
-
   Overloading the function @{text \<calL>} for the two kinds of terms in the
   equational system, we have
   
@@ -883,26 +888,23 @@
   then we calculate the combined regular expressions for all @{text r} coming 
   from the deleted @{text "(X, r)"}, and take the @{const Star} of it;
   finally we append this regular expression to @{text rhs'}. If we apply this
-  operation to the right-hand side of @{text "X\<^isub>3"} in \eqref{exmpcs}, we obtain
+  operation to the right-hand side of @{text "X\<^isub>4"} in \eqref{exmpcs}, we obtain
   the equation:
 
   \begin{center}
   \begin{tabular}{l@ {\hspace{1mm}}c@ {\hspace{1mm}}l}
-  @{term "X\<^isub>3"} & @{text "="} & 
-    @{text "(X\<^isub>1, TIMES (ATOM d\<^isub>1) (STAR \<^raw:\ensuremath{\bigplus}>{ATOM c\<^isub>1,\<dots>, ATOM c\<^isub>m})) + \<dots> "}\\
-  & & \mbox{}\hspace{13mm}
-     @{text "\<dots> + (X\<^isub>1, TIMES (ATOM d\<^isub>n) (STAR \<^raw:\ensuremath{\bigplus}>{ATOM c\<^isub>1,\<dots>, ATOM c\<^isub>m}))"}
+  @{term "X\<^isub>4"} & @{text "="} & 
+    @{text "(X\<^isub>1, TIMES (ATOM b) (STAR \<^raw:\ensuremath{\bigplus}>{ATOM a, ATOM b})) +"}\\
+  & & @{text "(X\<^isub>2, TIMES (ATOM a) (STAR \<^raw:\ensuremath{\bigplus}>{ATOM a, ATOM b})) +"}\\
+  & & @{text "(X\<^isub>3, TIMES (ATOM a) (STAR \<^raw:\ensuremath{\bigplus}>{ATOM a, ATOM b})) +"}\\
+  & & @{text "(X\<^isub>3, TIMES (ATOM b) (STAR \<^raw:\ensuremath{\bigplus}>{ATOM a, ATOM b}))"}
   \end{tabular}
   \end{center}
 
 
   \noindent
-  That means we eliminated the recursive occurrence of @{text "X\<^isub>3"} on the
-  right-hand side.  Note we used the abbreviation 
-  @{text "\<^raw:\ensuremath{\bigplus}>{ATOM c\<^isub>1,\<dots>, ATOM c\<^isub>m}"} 
-  to stand for a regular expression that matches with every character. In 
-  our algorithm we are only interested in the existence of such a regular expression
-  and do not specify it any further. 
+  That means we eliminated the recursive occurrence of @{text "X\<^isub>4"} on the
+  right-hand side. 
 
   It can be easily seen that the @{text "Arden"}-operation mimics Arden's
   Lemma on the level of equations. To ensure the non-emptiness condition of
@@ -951,7 +953,7 @@
   %
   \begin{equation}\label{exmpresult}
   \mbox{\begin{tabular}{l@ {\hspace{1mm}}c@ {\hspace{1mm}}l}
-  @{term "X\<^isub>2"} & @{text "="} & @{text "\<lambda>(TIMES ONE (ATOM c))"}
+  @{term "X\<^isub>2"} & @{text "="} & @{text "\<lambda>(TIMES ONE (ATOM a))"}
   \end{tabular}}
   \end{equation}
 
@@ -1194,7 +1196,15 @@
 
   \begin{proof}[of Theorem~\ref{myhillnerodeone}]
   By Lemma~\ref{every_eqcl_has_reg} we know that there exists a regular expression for
-  every equivalence class in @{term "UNIV // \<approx>A"}. Since @{text "finals A"} is
+  every equivalence class in @{term "UNIV // \<approx>A"}:
+
+  \[
+  \mbox{if}\;@{term "X \<in> (UNIV // \<approx>A)"}\;\mbox{then there exists a}\; 
+  @{text "r"}\;\mbox{such that}\;@{term "X = lang r"}
+  \]\smallskip
+
+  \noindent
+  Since @{text "finals A"} is
   a subset of  @{term "UNIV // \<approx>A"}, we also know that for every equivalence class
   in @{term "finals A"} there exists a regular expression. Moreover by assumption 
   we know that @{term "finals A"} must be finite, and therefore there must be a finite
@@ -1205,12 +1215,27 @@
   \end{proof}
 
   \noindent
+  Solving the equational system of our running example gives as solution for the 
+  two final equivalence classes:
+
+  \begin{center}
+  \begin{tabular}{r@ {\hspace{1mm}}c@ {\hspace{1mm}}l}
+  @{text "X\<^isub>2"} & @{text "="} & @{text "TIMES ONE (ATOM a)"}\\
+  @{text "X\<^isub>3"} & @{text "="} & @{text "TIMES (TIMES ONE (ATOM a)) (ATOM b)"}
+  \end{tabular}
+  \end{center}
+
+  \noindent
+  Combining them with $\bigplus$ gives us a regular expression for the language @{text "{[a], [a, b]}"}.
+
   Note that our algorithm for solving equational systems provides also a method for
   calculating a regular expression for the complement of a regular language:
   if we combine all regular
-  expressions corresponding to equivalence classes not in @{term "finals A"},
+  expressions corresponding to equivalence classes not in @{term "finals A"} 
+  (in the running example @{text "X\<^isub>1"} and @{text "X\<^isub>4"}),
   then we obtain a regular expression for the complement language @{term "- A"}.
   This is similar to the usual construction of a `complement automaton'.
+
 *}
 
 section {* Myhill-Nerode, Second Part *}
@@ -1296,10 +1321,13 @@
 
   \noindent
   For constructing @{text R}, we will rely on some \emph{tagging-functions}
-  defined over strings. Given the inductive hypothesis, it will be easy to
+  defined over strings, see Fig.~\ref{tagfig}. They are parameterised by sets
+  of strings @{text A} and @{text B} standing for the induction hypotheses.
+  Given the inductive hypotheses, it will be easy to
   prove that the \emph{range} of these tagging-functions is finite. The range
   of a function @{text f} is defined as
 
+ 
   \begin{center}
   @{text "range f \<equiv> f ` UNIV"}
   \end{center}
@@ -1334,6 +1362,24 @@
   is finite, then the set @{text A} itself must be finite. We can use it to establish the following 
   two lemmas.
 
+  \begin{figure}[t]
+  \normalsize
+  \begin{tabular}{l@ {\hspace{1mm}}c@ {\hspace{1mm}}l}
+  @{thm (lhs) tag_Plus_def[where A="A" and B="B", THEN meta_eq_app]} &
+  @{text "\<equiv>"} &
+  @{thm (rhs) tag_Plus_def[where A="A" and B="B", THEN meta_eq_app]} \\
+  @{thm (lhs) tag_Times_def[where ?A="A" and ?B="B"]}\;@{text "x"} & @{text "\<equiv>"} &
+  @{text "(\<lbrakk>x\<rbrakk>\<^bsub>\<approx>A\<^esub>, {\<lbrakk>x\<^isub>s\<rbrakk>\<^bsub>\<approx>B\<^esub> | x\<^isub>p \<in> A \<and> (x\<^isub>p, x\<^isub>s) \<in> Partitions x})"}\\
+  @{thm (lhs) tag_Star_def[where ?A="A", THEN meta_eq_app]} & @{text "\<equiv>"} &
+  @{text "{\<lbrakk>x\<^isub>s\<rbrakk>\<^bsub>\<approx>A\<^esub> | x\<^isub>p < x \<and> x\<^isub>p \<in> A\<^isup>\<star> \<and> (x\<^isub>p, x\<^isub>s) \<in> Partitions x}"}
+  \end{tabular}
+  \caption{Three tagging functions used the cases for @{term "PLUS"}, @{term "TIMES"} and @{term "STAR"} 
+  regular expressions. The sets @{text A} and @{text B} are arbitrary languages instantiated
+  in the proof with the induction hypotheses. \emph{Partitions} is a function
+  that generates all possible partitions of a string.\label{tagfig}}
+  \end{figure}
+
+
   \begin{lemma}\label{finone}
   @{thm[mode=IfThen] finite_eq_tag_rel}
   \end{lemma}
@@ -1379,7 +1425,7 @@
   Chaining Lemma~\ref{finone} and \ref{fintwo} together, means in order to show
   that @{term "UNIV // \<approx>(lang r)"} is finite, we have to construct a tagging-function whose
   range can be shown to be finite and whose tagging-relation refines @{term "\<approx>(lang r)"}.
-  Let us attempt the @{const PLUS}-case first. We take as tagging-function 
+  Let us attempt the @{const PLUS}-case first. We take as tagging-function from Fig.~\ref{tagfig}
  
   \begin{center}
   @{thm tag_Plus_def[where A="A" and B="B", THEN meta_eq_app]}
@@ -1510,7 +1556,7 @@
   tagging-function in the @{const Times}-case as:
 
   \begin{center}
-  @{thm (lhs) tag_Times_def[where ?A="A" and ?B="B"]}~@{text "\<equiv>"}~
+  @{thm (lhs) tag_Times_def[where ?A="A" and ?B="B"]}\;@{text "x"}~@{text "\<equiv>"}~
   @{text "(\<lbrakk>x\<rbrakk>\<^bsub>\<approx>A\<^esub>, {\<lbrakk>x\<^isub>s\<rbrakk>\<^bsub>\<approx>B\<^esub> | x\<^isub>p \<in> A \<and> (x\<^isub>p, x\<^isub>s) \<in> Partitions x})"}
   \end{center}
 
@@ -1679,6 +1725,58 @@
   A\<star>"}, which means @{term "y @ z \<in> A\<star>"}. The last step is to set
   @{text "A"} to @{term "lang r"} and thus complete the proof.
   \end{proof}
+
+  \begin{rmk}
+  While our proof using tagging functions might seem like a rabbit pulled 
+  out of a hat, the intuition behind can be rationalised taking the 
+  view that the equivalence classes @{term "UNIV // \<approx>(lang r)"} stand for the 
+  states of the minimal automaton for the language @{term "lang r"}. The theorem 
+  amounts to showing that this minimal automaton has finitely many states. 
+  However, by showing that our @{text "\<^raw:$\threesim$>\<^bsub>tag\<^esub>"} relation 
+  refines @{term "\<approx>A"} we do not actually have to show that the minimal automata
+  has finitely many states, but only that we can show finiteness for an 
+  automaton with at least as many states and which can recognise the same 
+  language. By performing the induction over the regular expression @{text r},
+  this means we have to construct inductively an automaton in
+  the cases for @{term PLUS}, @{term TIMES} and @{term STAR}.
+
+  In the @{text PLUS}-case, we can assume finitely many equivalence classes of the form
+  @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>A\<^esub>"} and @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>B\<^esub>"}, each standing for an automaton recognising the 
+  languages @{text A} and @{text B} respectively. 
+  The @{text "+tag"} function constructs pairs of the form @{text "(\<lbrakk>_\<rbrakk>\<^bsub>\<approx>A\<^esub>, \<lbrakk>_\<rbrakk>\<^bsub>\<approx>B\<^esub>)"} 
+  which can be seen as the states of an automaton recognising the language 
+  \mbox{@{term "A \<union> B"}}. This is the usual product construction of automata: 
+  Given a string @{text x}, the first automata will be in the state  @{text "\<lbrakk>x\<rbrakk>\<^bsub>\<approx>A\<^esub>"}
+  after recognising @{text x} (similarly @{text "\<lbrakk>x\<rbrakk>\<^bsub>\<approx>B\<^esub>"} for the other automaton). The product
+  automaton will be in the state \mbox{@{text "(\<lbrakk>x\<rbrakk>\<^bsub>\<approx>A\<^esub>, \<lbrakk>x\<rbrakk>\<^bsub>\<approx>B\<^esub>)"}}, which is accepting
+  if at least one component is an accepting state.
+
+  The @{text "TIMES"}-case is a bit more complicated---essentially we 
+  need to sequentially compose the two automata with the states @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>A\<^esub>"}, 
+  respectively @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>B\<^esub>"}. We achieve this sequential composition by
+  taking the first automaton @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>A\<^esub>"} and append on each of its accepting
+  state the automaton @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>B\<^esub>"}. Unfortunately, this does not lead directly
+  to a correct composition, since the automaton @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>A\<^esub>"} might have consumed
+  some of the input needed for the automaton @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>B\<^esub>"} to succeed. The 
+  textbook construction for sequentially composing two automata circumvents this
+  problem by connecting the terminating states of the first automaton via
+  an epsilon-transition to the initial state of the second automaton (see for
+  example \citeN{Kozen97}). In the absence of any epsilon-transition analogue in our work, 
+  we let the second automaton
+  start in all possible states that may have been reached if the first
+  automaton had not consumed the input meant for the second. We achieve this
+  by having a set of states as the second component generated by our 
+  @{text "\<times>tag"} function (see second clause in Fig.~\ref{tagfig}).
+  A state of this sequentially composed automaton is accepting, if the first 
+  component is accepting and at least one state in the set is also accepting.
+
+  The idea behind the @{text "STAR"}-case is similar to the @{text "TIMES"}-case.
+  We assume some automaton has consumed some strictly smaller part of the input;
+  we need to check that from the state we ended up in a terminal state in the 
+  automaton @{text "\<lbrakk>_\<rbrakk>\<^bsub>\<approx>A\<^esub>"}. Since we do not know from which state this will 
+  succeed, we need to run the automaton from all possible states we could have 
+  ended up in. Therefore the @{text "\<star>tag"} function generates a set of states.
+  \end{rmk}
 *}
 
 section {* Second Part proved using Partial Derivatives *}
@@ -2211,12 +2309,12 @@
   %
   \begin{equation}\label{supseqprops}
   \mbox{\begin{tabular}{l@ {\hspace{1mm}}c@ {\hspace{1mm}}l}
-  @{thm (lhs) SUPSEQ_simps(1)} & @{text "\<equiv>"} & @{thm (rhs) SUPSEQ_simps(1)}\\  
-  @{thm (lhs) SUPSEQ_simps(2)} & @{text "\<equiv>"} & @{thm (rhs) SUPSEQ_simps(2)}\\ 
-  @{thm (lhs) SUPSEQ_atom} & @{text "\<equiv>"} & @{thm (rhs) SUPSEQ_atom}\\ 
-  @{thm (lhs) SUPSEQ_union} & @{text "\<equiv>"} & @{thm (rhs) SUPSEQ_union}\\
-  @{thm (lhs) SUPSEQ_conc} & @{text "\<equiv>"} & @{thm (rhs) SUPSEQ_conc}\\
-  @{thm (lhs) SUPSEQ_star} & @{text "\<equiv>"} & @{thm (rhs) SUPSEQ_star}
+  @{thm (lhs) SUPSEQ_simps(1)} & @{text "="} & @{thm (rhs) SUPSEQ_simps(1)}\\  
+  @{thm (lhs) SUPSEQ_simps(2)} & @{text "="} & @{thm (rhs) SUPSEQ_simps(2)}\\ 
+  @{thm (lhs) SUPSEQ_atom} & @{text "="} & @{thm (rhs) SUPSEQ_atom}\\ 
+  @{thm (lhs) SUPSEQ_union} & @{text "="} & @{thm (rhs) SUPSEQ_union}\\
+  @{thm (lhs) SUPSEQ_conc} & @{text "="} & @{thm (rhs) SUPSEQ_conc}\\
+  @{thm (lhs) SUPSEQ_star} & @{text "="} & @{thm (rhs) SUPSEQ_star}
   \end{tabular}}
   \end{equation}
 
@@ -2365,24 +2463,9 @@
   exists a regular expression that matches all of its strings. Regular
   expressions can conveniently be defined as a datatype in theorem
   provers. For us it was therefore interesting to find out how far we can push
-  this point of view. But this question whether formal language theory can
-  be done without automata crops up also in non-theorem prover contexts. For 
-  example, Gasarch asked in the Computational Complexity blog by \citeN{GasarchBlog} 
-  whether the complementation of 
-  regular-expression languages can be proved without using automata. He concluded
- 
-  \begin{quote}
-  {\it \ldots it can't be done}
-  \end{quote} 
-
-  \noindent
-  and even pondered
-
-  \begin{quote}
-  {\it \ldots [b]ut is there a rigorous way to even state that?} 
-  \end{quote} 
-
-  \noindent
+  this point of view. But this question whether regular language theory can
+  be done without automata crops up also in non-theorem prover contexts. Recall 
+  Gasarch's speculation cited in the Introduction. 
   We have established in Isabelle/HOL both directions 
   of the Myhill-Nerode Theorem.
   %
@@ -2398,7 +2481,7 @@
   Pumping Lemma).  We can also use it to establish the standard
   textbook results about closure properties of regular languages. The case of 
   closure under complement follows easily from the Myhill-Nerode Theorem.
-  So our answer to Gasarch is ``{\it it can be done!''}  
+  So our answer to Gasarch is ``{\it yes we can!''}  
 
   %Our insistence on regular expressions for proving the Myhill-Nerode Theorem
   %arose from the problem of defining formally the regularity of a language.
@@ -2465,15 +2548,15 @@
   in the `pencil-and-paper-reasoning' literature about our way of proving the 
   first direction of the Myhill-Nerode Theorem, but it appears to be folklore.
 
-  We presented two proofs for the second direction of the Myhill-Nerode
-  Theorem. One direct proof using tagging-functions and another using partial
-  derivatives. This part of our work is where our method using regular
-  expressions shines, because we can completely side-step the standard
-  argument (for example used by \citeN{Kozen97}) where automata need to be composed. However, it is
-  also the direction where we had to spend most of the `conceptual' time, as
-  our first proof based on tagging-functions is new for establishing the
-  Myhill-Nerode Theorem. All standard proofs of this direction proceed by
-  arguments over automata.
+  We presented two proofs for the second direction of the
+  Myhill-Nerode Theorem. One direct proof using tagging-functions and
+  another using partial derivatives. This part of our work is where
+  our method using regular expressions shines, because we can perform
+  an induction on the structure of refular expressions. However, it is
+  also the direction where we had to spend most of the `conceptual'
+  time, as our first proof based on tagging-functions is new for
+  establishing the Myhill-Nerode Theorem. All standard proofs of this
+  direction proceed by arguments over automata.
   
   The indirect proof for the second direction arose from our interest in
   Brzozowski's derivatives for regular expression matching.  While \citeN{Brzozowski64} 




