1 (defun iterative-improve (good-enough? improve)
2   (lambda (first-guess)
3     (labels ((improve-iter (guess)
4 	       (let ((improved-guess (funcall improve guess)))
5 		 (if (funcall good-enough? guess improved-guess)
6 		     improved-guess
7 		     (improve-iter improved-guess)))))
8 	     (improve-iter first-guess))))
9 (defun improved-sqrt (num)
10   (funcall (iterative-improve
11 	    (lambda (x y)
12 	      (let ((ratio (/ x y)))
13 		(and (< ratio 1.001) (> ratio 0.999))))
14 	    (lambda (guess)
15 	      (average guess (/ num guess))))
16 	   1.0))
17 (defvar *tolerance* 0.00001)
18 (defun improved-fixed-point (f first-guess)
19   (funcall (iterative-improve
20 	    (lambda (x y)
21 	      (< (abs (- x y)) *tolerance*))
22 	    (lambda (guess)
23 	      (funcall f guess)))
24 	   first-guess))
25