(define fast-sqrt
  ; TYPE: (-> (number) number)
  (lambda (a)
    ; REQUIRES: a > 0
    ; ENSURES: result is an approx
    ; to the square root of a
    (let ((u 1.0)
	  (improve
	   (lambda (u)
	     (/ (+ u (/ a u)) 2))))
      (set! u (improve u))
      (set! u (improve u))
      (set! u (improve u))
      u)))