sicp exercise 3.28

; Exercise 3.28.  Define an or-gate as a primitive function box. Your or-gate constructor should be similar to and-gate.

(define (or-gate a b out)
  (define (or-action-procedure)
    (let ((new-value (logical-or (get-signal a) (get-signal b))))
       (after-delay or-gate-delay
                    (lambda()
                      (set-signal! out new-value)))))
  (add-action! a or-action-procedure)
  (add-action! a or-action-procedure))