sicp-3-3-5

(define (average a b c)
  (let ((x (make-connector))
        (y (make-connector)))
    (adder a b x)
    (multiplier c v u)
    (constant 2 v))
  'ok)

(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! b me)
    (forget-value! a me))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)
           (process-new-value))
          ((eq? request 'I-lost-my value)
           (process-new-value))
          (else
            (error "Unkown request  SQUARE" request))))
  (connect a me)
  (connect b me)
  me)

(define (c- x y)
  (let ((z (make-connector)))
    (adder x y z)
    z))

(define (c* x y)
  (let ((z (make-connector)))
    (multiplier x y z)
    z))

(define (c/ x y)
  (let ((z (make-connector)))
    (multiplier y z x)
    z))

(define (cv x)
  (let ((z (make-connector)))
    (constant x z)
    z))