functor Double_Stack_Queue_Fn (myStack : Stack) :> Queue =
  struct
    type 'a queue = ('a myStack.stack, 'a myStack.stack)
    exception Empty
    val empty = (myStack.empty, myStack.empty)
    fun enqueue (f, s) e = (myStack.push f e, s)
    fun dequeue (f, s) = case s of
                             [] => if f = myStack.empty then raise Empty else dequeue (myStack.empty, rev f)
                           | x::xs => (x, (f, xs))
  end