\documentclass{article}
\usepackage{program}
\begin{document}
\begin{programbox}
\mbox{A fast exponential procedure:}
\BEGIN
\FOR i := 1 \TO 10 \STEP 1 \DO
|expt|(2,i);
|newline|
\OD
\WHERE \rcomment{A comment here}
\PROC |expt|(x,n) \BODY
z := 1;
\DO \IF n = 0 \THEN \EXIT \FI;
\DO \IF |odd|(n) \THEN \EXIT \FI;
n := n/2; x := x*x;
\OD;
n := n-1;
z := z*x
\OD;
|print|(z) \ENDPROC
\END
\end{programbox}
\end{document}