structure IntStack : QueueSig =
struct
    exception Deq
    type Item = int
    abstype Queue = Q of Item 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;