; queue = (cons in out)
(define (make-queue) '(() . ()))

; writing: just push the in stack
(define (queue-write! queue thing)
  (set-car! queue (cons  thing (car queue))))

; reading: if out stack is zero, pop-push in to out
(define (queue-read! queue)
  (if (null? (cdr queue))
      (begin
	(if (null? (car queue)) (throw 'queue-empty))
	(set-cdr! queue (reverse! (car queue)))
	(set-car! queue '())
	(queue-read! queue))
      (let ((thing (cadr queue)))
	(set-cdr! queue (cddr queue))
	thing)))