연습문제 3.35

(define (squarer a b)
(define (process-new-value)
(if (has-value? b)
(if (< (get-value b ) 0)
(error "square less than 0 -- SQUARER" (get-value b))
(set-value! a (sqrt (get-value b)) me))
(if (has-value? a)
(set-value! b (square (get-value a)) me) )))
(define (process-forget-value)
(forget-value! a me)
(forget-value! b me))
(define (me request)
(cond ((eq? request 'I-have-a-value)
(process-new-value))
((eq? request 'I-lost-my-value)
(process-forget-value))
(else
(error "Unknown request -- SQUARER" request))))
(connect a me)
(connect b me)
me)