;;Ex.3.30
;;a-wire, b-wire, s-wire are lists of n wires
(define (ripple-carry-adder a-wire b-wire s-wire c)
  (define (get-carry a b s) ;a,b,s are lists of wires
    (if (null? a)
        (let ((d (make-wire))) ;initial carry-in value of 0
          (set-signal! d 0)
          d)
        (let ((e (make-wire))) ;the carry wire
          (full-adder (car a) (car b)
                      (get-carry (cdr a) (cdr b) (cdr s)) (car s) e)
          e))) ;return it

  (full-adder (car a-wire) (car b-wire)
              (get-carry (cdr a-wire) (cdr b-wire) (cdr s-wire))
              (car s-wire) c)
  'ok)