exception Empty;

fun newStack (x) =
    let
	val store = ref [x]
    in
	{push = 
	   fn y => store := y::(!store),
	 pop = 
	   fn () => case !store of
			nil => raise Empty
		      | (y::ys) => (store := ys; 
				    y)
	 }
    end;

val s = newStack(0);
#push(s)(1);
#pop(s)();
#pop(s)();

fun newCountingStack (x) =
    let
	val store = ref [x]
	val counter = ref 1
    in
	{push = 
	   fn y => (counter := !counter + 1;
		    store := y::(!store)),
	 pop = 
	   fn () => case !store of
			nil => raise Empty
		      | (y::ys) => (counter := !counter - 1;
				    store := ys; 
				    y),
	 counter = 
	   fn () => !counter
	 }
    end;

val cs = newCountingStack(0);
#push(cs)(1);
#counter(cs)();
#pop(cs)();
#pop(cs)();
#counter(cs)();

(*
val l : {pop:unit -> int, push:int -> unit} = [s,cs]
*)