3.28

(define (or-gate a b output)  (define (or-action)    (let ((new-value (logic-or (get-signal a) (get-signal b))))      (after-delay or-gate-delay (lambda () (set-signal! output new-value)))))  (add-action! a or-action)  (add-action! b or-action)  'ok)

3.29

a ∨ b == ~~(a ∨ b) == ~(~a ∧ ~b)

(define (or-gate a b output)  (let ((a-out (make-wire))        (b-out (make-wire))        (and-out (make-wire)))    (inverter a a-out)    (inverter b b-out)    (and-gate a-out b-out and-out)    (inverter and-out output)))

3.30

(define (ripple-carry-adder a b s c)  (if (null? (cdr a))      (let ((cn (make-wire)))        (full-adder (car a) (car b) cn (car s) c))      (let ((ck (make-wire)))        (ripple-carry-adder (cdr a) (cdr b) (cdr s) ck)        (full-adder (car a) (car b) ck (car s) c))))