Sometimes a stopping criterion other than a simple counter is used. For example, we could test whether an answer is ``good enough'':

(define (mysqrt x) (mysqrtb x 1.0 1.0e-8))

(define (mysqrtb x estimate epsilon)
  (if (good-enough x (square estimate) epsilon)
      estimate
      (mysqrtb x (new-estimate x estimate)
                 epsilon) ) )

(define (square x) (* x x))

(define (good-enough x y epsilon)
  (< (abs (- x y)) epsilon))

(define (new-estimate x estimate)
  (/ (+ estimate
        (/ x estimate))
     2))

The question of whether a program will terminate is an important one; sometimes it is difficult to answer.

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