1
\begin{isabelle}%
2
~{"}let~(x,y)~=~f~z~in~(y,x){"}~{"}case~xs~of~[]~{\isasymRightarrow}~0~|~(x,y)\#zs~{\isasymRightarrow}~x+y{"}\isanewline
3
\end{isabelle}%