functor PQUEUE(type Item 
               val > : Item * Item -> bool
              ):QueueSig =
struct
    type Item = Item
    exception Deq
    fun insert e [] = [e]:Item list
      | insert e (h :: t) = 
        if e > h then e :: h :: t
                 else h :: insert e t
                          
    abstype Queue = Q of Item list
    with
        val empty          = Q []
        fun isEmpty (Q []) = true 
          | isEmpty _      = false
 
        fun enq(Q q, e)    = Q(insert e q)
        fun deq(Q(h :: t)) = (Q t, h)
          | deq _          = raise Deq
    end
end;