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

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(define (or-gate in1 in2 out)
  (define (update)
    (let ((new-value (logical-or (get-signal in1) (get-signal in2))))
      (after-delay or-gate-delay
        (lambda ()
          (set-signal! out new-value)))))
  (add-action! in1 update)
  (add-action! in2 update)
  'ok)