1 (defun product (term a next b)
2   (if (> a b)
3       1
4     (* (funcall term a)
5        (product term (funcall next a) next b))))
6 (defun factorial (n)
7   (product #'identity 1 #'1+ n))
8 (defun wallis-pi (n)
9   (defun wallis-term (k)
10     (let ((nom
11 	   (if (evenp k)
12 	       (+ k 2)
13 	     (+ k 1)))
14 	  (denom
15 	   (if (evenp k)
16 	       (+ k 1)
17 	     (+ k 2))))
18       (float (/ nom denom))))
19   (* (product #'wallis-term 1 #'1+ n)))
20 
21 (defun product-iter (term a next b)
22   (defun iter (a result)
23     (if (> a b)
24 	result
25       (iter (funcall next a)
26 	    (* (funcall term a) result))))
27   (iter a 1))
28