structure Stack =
struct
    exception Deq;
    type Item = int
    abstype Queue = Q of int list
    with
        val empty           = Q []

        fun isEmpty (Q [])  = true 
          | isEmpty _       = false

        fun enq(Q s, e)     = Q(e :: s)

        fun deq (Q(h :: t)) = (Q t, h)
          | deq  _          = raise Deq
    end 
end;