1 (defun accumulator (combiner null-value term a next b)
2   (if (> a b)
3       null-value
4     (funcall combiner
5 	     (funcall term a)
6 	     (accumulator combiner null-value term (funcall next a) next b))))
7 (defun sum (term a next b)
8   (accumulator #'+ 0 term a next b))
9 (defun accumulator-iter (combiner null-value term a next b)
10   (defun iter (a result)
11     (if (> a b)
12 	result
13       (iter (funcall next a)
14 	    (funcall combiner (funcall term a) result))))
15   (iter a null-value))
16 (defun product (term a next b)
17   (accumulator-iter #'* 1 term a next b))