MODULE Showcase13;
// Step showcase: FORWARD declarations driving mutual recursion, plus
// BITSET set operations (union, membership) and the typed set
// constructor BITSET{...}. Prints a result and exits 42.
IMPORT SysIO;

VAR ExitCode : INTEGER;
VAR s, t : BITSET;
VAR n : INTEGER;

// --- mutual recursion via FORWARD -------------------------------
PROCEDURE Even (k : INTEGER) : BOOLEAN; FORWARD;
PROCEDURE Odd  (k : INTEGER) : BOOLEAN; FORWARD;

PROCEDURE Even (k : INTEGER) : BOOLEAN;
BEGIN
  IF k = 0 THEN RETURN TRUE ELSE RETURN Odd(k - 1) END
END Even;

PROCEDURE Odd (k : INTEGER) : BOOLEAN;
BEGIN
  IF k = 0 THEN RETURN FALSE ELSE RETURN Even(k - 1) END
END Odd;

PROCEDURE Show (label : ARRAY OF CHAR; v : INTEGER);
BEGIN
  SysIO.Write(label);
  SysIO.WriteInt(v);
  SysIO.WriteLn
END Show;

BEGIN
  n := 0;
  IF Even(8) THEN n := n + 10 END;    // TRUE  -> 10
  IF Odd(7)  THEN n := n + 11 END;    // TRUE  -> 21

  s := BITSET{1, 3, 5};               // typed set constructor
  t := BITSET{5, 7};
  IF 3 IN s THEN n := n + 10 END;     // member -> 31

  s := s + t;                         // union {1, 3, 5, 7}
  IF 7 IN s THEN n := n + 11 END;     // member -> 42

  Show("Even(8)=T Odd(7)=T total=", n);
  ExitCode := n
END Showcase13.