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 ;;or-gate implementation  : 
 (define (or-gate a1 a2 output) 
   (let ((c (make-wire)) 
         (d (make-wire)) (e (make-wire)) 
         (f (make-wire) (g (make-wire)))) 
     (and-gate a1 a1 d) 
     (and-gate a2 a2 e) 
     (inverter d f) 
     (inverter e g) 
     (and-gate f g c) 
     (inverter c output) 
 ;; a 
 ;; (A or B) is equivalent to (not ((not A) and (not B))) 
 (define (or-gate a1 a2 output) 
   (let ((c1 (make-wire)) 
         (c2 (make-wire)) 
         (c3 (make-wire))) 
     (inverter a1 c1) 
     (inverter a2 c2) 
     (and-gate c1 c2 c3) 
     (inverter c3 output))) 
 ;; b 
 the delay is the sum of and-gate-delay plus twice inverter-delay.