1 (define (ripple-carry-adder A B S C)
2   (let ((c-in (make-wire)))
3     (if (null? (cdr A))
4 	(set-signal! c-in 0)
5 	(ripple-carry-adder (cdr A) (cdr B) (cdr S) c-in))
6     (full-adder (car A) (car B) c-in (car S) C)))