functor RQUEUE( type Item ):QueueSig =
struct
    exception Deq
    type Item = Item
    abstype Queue = 
       Q of (Item list * Item list) ref
    with
        val empty         = Q (ref ([], []))
        fun isEmpty(Q (ref([], []))) = true 
          | isEmpty _                = false
 
        fun enq(Q(ref (inp, out)), e) = 
                Q(ref(e :: inp, out))
        fun deq(Q(ref(inp, h :: t))) = 
               (Q(ref(inp, t)), h)
          | deq(Q(ref([], []))) = raise Deq
          | deq(Q(r as ref(inp, []))) = 
               (r := ([], rev inp); deq(Q r))
    end
end;