GENERIC IMPLEMENTATION MODULE GTwice;
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;
END GTwice.