\begin{center}\begin{minipage}{15cm}\begin{Verbatim}[frame=single]
> fac = proc(n) { var res; if (n == 0) then res := 1 else res := n * fac(n - 1);
 return res; };
> fac(5);
120
> fac(11);
39916800
> fac;
proc(n)
{
var res;
if (n) == (0) then
res := 1
else
res := (n) * (fac((n) - (1)));
return res;
}
\end{Verbatim}
\end{minipage}\end{center}
