1 ;; weiqun zhang's solution
2 
3 (define (make-queue)
4   (let ((front-ptr '())
5 	(rear-ptr '()))
6     (define (prt-q)
7       (newline)
8       (display front-ptr))
9     (define (emp-q?)
10       (null? front-ptr))
11     (define (fro-q)
12       (if (emp-q?)
13 	  (error "FRONT called with an empty queue")
14 	  (car front-ptr)))
15     (define (ins-q! item)
16       (let ((new-pair (cons item '())))
17 	(cond ((emp-q?)
18 	       (set! front-ptr new-pair)
19 	       (set! rear-ptr new-pair))
20 	      (else
21 	       (set-cdr! rear-ptr new-pair)
22 	       (set! rear-ptr new-pair)))))