(define (read key lis)
 (call/cc (lambda (return)
  (fold (lambda (cur prev)
   (if (eq? key (car cur))
    (return (car (cdr cur)))
    prev)) #f lis))))

(define (write key val lis)
 (if (eq? #f (read key lis))
  (cons (list key val) lis)
  (map (lambda (cur)
   (if (eq? (car cur) key)
    (list key val)
    cur)) lis)))