Exercise 3.29.  Another way to construct an or-gate is as a compound 
digital logic device, built from and-gates and inverters. Define a 
procedure or-gate that accomplishes this. What is the delay time of the 
or-gate in terms of and-gate-delay and inverter-delay?

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(define (or-gate in1 in2 out)
  (define a (make-wire))
  (define b (make-wire))
  (define c (make-wire))
  (inverter in1 a)
  (inverter in2 b)
  (and-gate a b c)
  (inverter c out))

The delay is and-gate-delay + 2 × inverter-delay.