Ex-3.6

(define rand-init 10)
(define rand
  (let ((next rand-init))
    (define (dispatch msg)
      (cond
       ((eq? msg 'generate)
        (set! next (rand-update init))
        next)
       ((eq? msg 'reset)
        (lambda (new-val) (set! next new-val)))
       (else (error "message not recognized"))))
    dispatch))