MODULE PFwd;
VAR ExitCode : INTEGER;
PROCEDURE Even(n : INTEGER) : BOOLEAN;
FORWARD;
PROCEDURE Odd(n : INTEGER) : BOOLEAN;
BEGIN
  IF n = 0 THEN RETURN FALSE ELSE RETURN Even(n - 1) END
END Odd;
PROCEDURE Even(n : INTEGER) : BOOLEAN;
BEGIN
  IF n = 0 THEN RETURN TRUE ELSE RETURN Odd(n - 1) END
END Even;
BEGIN
  IF Even(10) THEN ExitCode := 1 ELSE ExitCode := 0 END
END PFwd.