1 (define (front-ptr queue) (car queue))
2 (define (rear-ptr queue) (cdr queue))
3 (define (set-front-ptr! queue item) (set-car! queue item))
4 (define (set-rear-ptr! queue item) (set-cdr! queue item))
5 
6 (define (empty-queue? queue) (null? (front-ptr queue)))
7 (define (make-queue) (cons '() '()))
8 (define (front-queue queue)
9   (if (empty-queue? queue)
10       (error "FRONT called with an empty queue" queue)
11       (car (front-ptr queue))))
12 
13 (define (insert-queue! queue item)
14   (let ((new-pair (cons item '())))
15     (cond ((empty-queue? queue)
16            (set-front-ptr! queue new-pair)
17            (set-rear-ptr! queue new-pair)
18            queue)
19           (else
20            (set-cdr! (rear-ptr queue) new-pair)
21            (set-rear-ptr! queue new-pair)
22            queue)))) 
23 (define (delete-queue! queue)
24   (cond ((empty-queue? queue)
25          (error "DELETE! called with an empty queue" queue))
26         (else
27          (set-front-ptr! queue (cdr (front-ptr queue)))
28          queue)))