--- a/handouts/ho05.tex	Sat Oct 26 01:02:59 2013 +0100
+++ b/handouts/ho05.tex	Sat Oct 26 01:22:21 2013 +0100
@@ -228,14 +228,25 @@
 \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. 
+expression cannot match anything at all. The mathematical way of stating this
+is 
 
 \begin{center}
-\texttt{\Grid{$c_1$\VS{} true\VS{} then\VS{} x\VS{} else\VS{} y \ldots }}
+$s \in zeroable(s)$ implies $L(r) = \varnothing$
 \end{center}
 
 \noindent
-The crucial idea of the algorithm is to build the derivative of all 
+Let us fix a set of regular expressions $rs$.
+The crucial idea of the algorithm is to take the input string, say
+
+\begin{center}
+\texttt{\grid{$c_1$}\grid{$c_2$}\grid{$c_3$}\grid{$c_4$}\grid{\ldots}}
+\end{center}
+
+\noindent
+and build the derivative of all regular expressions in $rs$ with respect
+to the first character. Then we take the result and continue with $c_2$
+until we have either exhausted our input string or all of the regula
 
 \end{document}