sicp-ex-3.30

  
  
 ;;a 
 (define (ripple-carry-adder a b s c) 
   (let ((c-in (make-wire))) 
         (if (null? (cdr a)) 
           (set-signal! c-in 0) 
           (ripple-carry-adder (cdr a) (cdr b) (cdr s) c-in)) 
         (full-adder (car a) (car b) c-in (car s) c)))