(define (sqrt x)
  (define tolerance 0.00001)
  (define (good-enough? y)
    (< (abs (- (* y y) x)) tolerance))
  (define (improve y)
    (average (/ x y) y))
  (define (try y)
    (if (good-enough? y)
	y
	(try (improve y))))
  (try 1))
(sqrt 13)