\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}