abstype 'elem stack =
    Empty |
    Body of 'elem list
with
    exception EmptyStack;

    val newStack = Empty;                       (* Initializer *)

    fun isEmpty(Empty) = true |
        isEmpty(Body(s)) = false;

    fun push(Empty, e) = Body[e] |              (* Constructor *)
        push(Body(s), e) = Body(e::s);

    fun pop(Empty) = Empty |                    (* Destructor *)
        pop(Body(nil)) = Empty |
        pop(Body(e::nil)) = Empty |
        pop(Body(e::es)) = Body(es);

    fun top(Empty) = raise EmptyStack |         (* Selector *)
        top(Body(s)) = hd(s);
end;