sicp-ex-3.28

  
  
  
 (define (or-gate a1 a2 output) 
   (define (or-action-procedure) 
         (let ((new-value  
                 (logic-or (get-singal 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)) 
 (define (logic-or s1 s2) 
   (if (or (= s1 1) (= s2 1)) 
         1 
         0))