GENERIC IMPLEMENTATION MODULE GStack;
PROCEDURE Init(VAR s : Stack);
BEGIN
  s.top := NIL; s.depth := 0
END Init;

PROCEDURE Push(VAR s : Stack; v : T);
VAR n : Node;
BEGIN
  NEW(n); n^.val := v; n^.next := s.top; s.top := n;
  s.depth := s.depth + 1
END Push;

PROCEDURE Pop(VAR s : Stack; VAR v : T) : BOOLEAN;
VAR n : Node;
BEGIN
  IF s.top = NIL THEN RETURN FALSE END;
  n := s.top; v := n^.val; s.top := n^.next;
  s.depth := s.depth - 1;
  RETURN TRUE
END Pop;

PROCEDURE Depth(VAR s : Stack) : INTEGER;
BEGIN
  RETURN s.depth
END Depth;
END GStack.