# 問題3.28

```(define a (make-wire))
(define b (make-wire))
(define c (make-wire))
(define d (make-wire))
(define e (make-wire))
(define s (make-wire))

(or-gate a b d)
(and-gate a b c)
(inverter c e)
(and-gate d e s)

(define (half-adder a b s c)
(let ((d (make-wire)) (e (make-wire)))
(or-gate a b d)
(and-gate a b c)
(inverter c e)
(and-gate d e s)
'ok))

(define (full-adder a b c-in sum c-out)
(let ((s (make-wire))
(c1 (make-wire))
(c2 (make-wire)))
(or-gate c1 c2 c-out)
'ok))

(define (logical-not s)
(cond ((= s 0) 1)
((= s 1) 0)
(else (error "Invalid signal" s))))
(define (inverter input output)
(define (invert-input)
(let ((new-value (logical-not (get-signal input))))
(after-delay inverter-delay
(lambda ()
(set-signal! output new-value)))))
'ok)

(define (logical-and x y)
(if (and (number? x) (number? y))
(if (and (= x 1) (= y 1)) 1 0)
(error "IN LOGICAL-AND: Invalid signal.")))
(define (and-gate a1 a2 output)
(define (and-action-procedure)
(let ((new-value
(logical-and (get-signal a1) (get-signal a2))))
(after-delay and-gate-delay
(lambda ()
(set-signal! output new-value)))))
'ok)
```
• 問題
```(define (logical-or x y)
(if (and (number? x) (number? y))
(if (or (= x 1) (= y 1)) 1 0)
(error "IN LOGICAL-OR: Invalid signal.")))
(define (or-gate a1 a2 output)
(define (or-action-procedure)
(let ((new-value
(logical-or (get-signal a1) (get-signal a2))))
(after-delay or-gate-delay
(lambda ()
(set-signal! output new-value)))))