structure Queue1: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 q, e)     = Q(q @ [e])

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