Exercise 3.28. Define an or-gate as a primitive function box. Your or-gate constructor should be similar to and-gate.
仿照 and-gate，我们可以定义 or-gate
(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! a1 or-action-procedure)
(add-action! a2 or-action-procedure)
'ok)