Type stack [int]
  declare
  	constructors
  		new () -> stack;
  		push (int, stack) -> stack;
  	destructors
  		pop (stack) -> stack;
  	selectors
  		top (stack) -> int;
  		isnew (stack) -> bool;
  for all
  	i in int;
  	s in stack;
  let
  	pop (new()) = error;
  	pop (push (i,s)) = s;
  	top (new()) = error;
  	top (push (i,s)) = i;
  	isnew (new()) = true;
  	isnew (push(i,s)) = false;
  end
end stack.