506   \mbox{}\hfill[1 Mark]

   506   \mbox{}\hfill[1 Mark]

   507 \end{itemize}

   507 \end{itemize}

   508 

   508 

   509 \subsection*{Background}

   509 \subsection*{Background}

   510 

   510 

   512 the constructors \texttt{ALT}/\texttt{SEQ}/\texttt{AND} needs a bit more thinking), the idea behind the

   512 the constructors \texttt{ALT}/\texttt{SEQ}/\texttt{AND} needs a bit more thinking), the idea behind the

   513 derivative function might not so easy to be seen. To understand its

   513 derivative function might not so easy to be seen. To understand its

   514 purpose better, assume a regular expression $r$ can match strings of

   514 purpose better, assume a regular expression $r$ can match strings of

   515 the form $c\!::\!cs$ (that means strings which start with a character

   515 the form $c\!::\!cs$ (that means strings which start with a character

   516 $c$ and have some rest, or tail, $cs$). If you take the derivative of

   516 $c$ and have some rest, or tail, $cs$). If you take the derivative of