;; boxed-value fns
(define (make-box value)
  (cons 'box value))

(define (box? obj)
  (eq? (car obj) 'box))

(define (box-get box)
  (cdr box))

(define (box-set! box value)
  (set-cdr! box value))

;; some fn that modifies its parameter
(define (foo box)
  (box-set! box 'modified))

;; test
(let1 box (make-box 42)
  (print (box-get box))
  (foo box)
  (print (box-get box)))

;; todo can one simulate dynamic scope with boxes?