(struct stack ([items #:auto]) #:mutable #:auto-value '())
(define (push! x stack)
  (set-stack-items! stack (cons x (stack-items stack))))
(define (pop! stack)
  (begin0 (car (stack-items stack))
    (set-stack-items! stack (cdr (stack-items stack)))))
(define (empty? stack)
  (null? (stack-items stack)))