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diff -r 95eaee695636 -r b6eee9571a63 handouts/ho05.tex
--- a/handouts/ho05.tex	Sat Oct 26 00:49:14 2013 +0100
+++ b/handouts/ho05.tex	Sat Oct 26 01:02:59 2013 +0100
@@ -210,7 +210,33 @@
 we can give the regular expression for \ref{Page ??} as follows
 
 Let us see how our algorithm for lexing works in detail. The regular 
-expressions and their ranking are shown above. 
+expressions and their ranking are shown above. For our algorithm it will  
+be helpful to have a look at the function \emph{zeroable}
+defined as follows:
+
+\begin{center}
+\begin{tabular}{@ {}l@ {\hspace{2mm}}c@ {\hspace{2mm}}l@ {}}
+$zeroable(\varnothing)$      &amp; $\dn$ &amp; $true$\\
+$zeroable(\epsilon)$           &amp; $\dn$ &amp;  $f\!alse$\\
+$zeroable (c)$                    &amp; $\dn$ &amp;  $f\!alse$\\
+$zeroable (r_1 + r_2)$        &amp; $\dn$ &amp;  $zeroable(r_1) \wedge zeroable(r_2)$ \\ 
+$zeroable (r_1 \cdot r_2)$  &amp; $\dn$ &amp;  $zeroable(r_1) \vee zeroable(r_2)$ \\
+$zeroable (r^*)$                  &amp; $\dn$ &amp; $f\!alse$\\
+\end{tabular}
+\end{center}
+
+\noindent
+In contrast to the function $nullable(r)$, which test whether a regular expression
+can match the empty string, the $zeroable$ function identifies whether a regular
+expression cannot match anything at all. 
+
+\begin{center}
+\texttt{\Grid{$c_1$\VS{} true\VS{} then\VS{} x\VS{} else\VS{} y \ldots }}
+\end{center}
+
+\noindent
+The crucial idea of the algorithm is to build the derivative of all 
+
 \end{document}
 
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