MODULE reverse;

   IMPORT Out, Strings;

   VAR s: ARRAY 12 + 1 OF CHAR;

   PROCEDURE Swap(VAR c, d: CHAR);
      VAR oldC: CHAR;
   BEGIN
      oldC := c; c := d; d := oldC
   END Swap;


   PROCEDURE Reverse(VAR s: ARRAY OF CHAR);
      VAR len, i: INTEGER;
   BEGIN
      len := Strings.Length(s);
      FOR i := 0 TO len DIV 2 DO
         Swap(s[i], s[len - 1 - i])
      END
   END Reverse;

BEGIN
   s := "hello, world";
   Reverse(s);
   Out.String(s);
   Out.Ln
END reverse.