(* Stack.sml *) open List; type 'a t = 'a list ref; exception Empty; fun new() = ref []; fun push x s = (s := x :: !s); fun pop s =  case !s of      [] => raise Empty    | x :: xs => (s := xs; x); fun peek s =  case !s of      [] => raise Empty    | x :: xs => x; fun update x s =  case !s of      [] => raise Empty    | _ :: xs => (s := x :: xs); fun null s = List.null (!s); fun clear s = (s := []);