SICP 問題 3.29

(define (or-gate a1 a2 output)
  (let ((b1 (make-wire))
        (b2 (make-wire))
        (c (make-wire)))
    (inverter a1 b1)
    (inverter a2 b2)
    (and-gate b1 b2 c)
    (inverter c output)))

;; 遅延時間は(+ and-gate-delay (* 2 inverter-delay))