Exercise 3.35.  Ben Bitdiddle tells Louis that one way to avoid the 
trouble in exercise 3.34 is to define a squarer as a new primitive 
constraint. Fill in the missing portions in Ben's outline for a 
procedure to implement such a constraint:

(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))
            <alternative1>)
        <alternative2>))
  (define (process-forget-value) <body1>)
  (define (me request) <body2>)
  <rest of definition>
  me)

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(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))))
        (set-value! b (square (get-value a)))))
  (define (process-forget-value)
    (forget-value! a)
    (forget-value! b)
    (process-new-value)) ;; required?
  (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" request))))
  (connect a me)
  (connect b me)
  me)