연습문제 3.28

(define (or-gate a1 a2 output)
  (define (or-action-procedure)
    (let ((new-value (logical-or (get-signal a1) (get-signal a2))))
      (after-delay or-gate-delay
                   (lambda ()
                     set-signal! output new-value))))
  (add-action! a2 or-action-procedure)
  (add-action! a1 or-action-procedure)
  'ok)