(define (accumulate-rec combiner null-value term a next b)
  (if (> a b)
      null-value
      (combiner (term a)
		(accumulate combiner null-value term (next a) next b))))

(define (accumulate-iter combiner null-value term a next b)
  (define (iter a result)
    (if (> a b)
	result
	(iter (next a) (combiner result (term a)))))
  (iter a null-value))

;(define accumulate accumulate-rec)
(define accumulate accumulate-iter)

(define (product term a next b)
  (accumulate 
    (lambda (x y) (* x y))
    1
    term
    a
    next
    b))

(define (sum term a next b)
  (accumulate
    (lambda (x y) (+ x y))
    0
    term
    a
    next
    b))

(define (fact n)
  (product
    (lambda (x) x)
    1
    (lambda (x) (+ x 1))
    n))