structure Queue2:QueueSig =
struct
    exception Deq
    type Item = int
    abstype Queue = Q of (Item list * Item list)
    with
        val empty              = Q ([], [])
        fun isEmpty(Q ([], []))= true 
          | isEmpty _          = false
 
        fun enq(Q(inp, out), e) = 
                               Q(e :: inp, out)

        fun deq(Q(inp, h :: t)) = (Q(inp, t), h)
          | deq(Q([], []))      = raise Deq
          | deq(Q(inp, []))     = deq(Q([], rev inp))
    end
end;