Exercise 3.22.  Instead of representing a queue as a pair of pointers, 
we can build a queue as a procedure with local state. The local state 
will consist of pointers to the beginning and the end of an ordinary 
list. Thus, the make-queue procedure will have the form

(define (make-queue)
  (let ((front-ptr ...)
        (rear-ptr ...))
    <definitions of internal procedures>
    (define (dispatch m) ...)
    dispatch))

Complete the definition of make-queue and provide implementations of the 
queue operations using this representation.

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(define (make-queue)
  (let ((front-ptr '())
        (rear-ptr '()))
    (define (insert item)
      (set-cdr! rear-ptr (list item))
      (set! rear-ptr (cdr rear-ptr)))
    (define (empty?)
      (null? front-ptr))
    (define (front)
      (if (empty?)
          (error "empty queue")
          front-ptr))
    (define (delete)
      (if (empty?)
          (error "empty queue")
          (set! front-ptr (cdr front-ptr))))
    (define (dispatch m)
      (cond ((eq? m 'insert-queue!) insert)
            ((eq? m 'delete-queue!) delete)
            ((eq? m 'empty-queue?) empty?)
            ((eq? m 'front-queue) front)
            (else (error "eh?"))))
    dispatch))