231 expression cannot match anything at all. 

   231 expression cannot match anything at all. The mathematical way of stating this

   232 

   232 is 

   233 \begin{center}

   233 

   234 \texttt{\Grid{$c_1$\VS{} true\VS{} then\VS{} x\VS{} else\VS{} y \ldots }}

   234 \begin{center}

   235 \end{center}

   235 $s \in zeroable(s)$ implies $L(r) = \varnothing$

   236 

   236 \end{center}

   237 \noindent

   237 

   238 The crucial idea of the algorithm is to build the derivative of all 

   238 \noindent

   239 Let us fix a set of regular expressions $rs$.

   240 The crucial idea of the algorithm is to take the input string, say

   241 

   242 \begin{center}

   243 \texttt{\grid{$c_1$}\grid{$c_2$}\grid{$c_3$}\grid{$c_4$}\grid{\ldots}}

   244 \end{center}

   245 

   246 \noindent

   247 and build the derivative of all regular expressions in $rs$ with respect

   248 to the first character. Then we take the result and continue with $c_2$

   249 until we have either exhausted our input string or all of the regula