structure Queue1:QueueSig =
struct
    abstype 'a Queue = Q of 'a list
    with
        val empty = Q []
        fun isEmpty (Q []) = true | isEmpty _ = false
 
        fun enq(Q q, e) = Q(e :: q)

        fun deq(Q [a]) = (Q [],a)
          | deq(Q (h :: t)) =
              let val (Q rest, last) = deq(Q t)
              in  (Q (h :: rest), last)
              end
     end
end