(define (sum-iter term a next b)
  (define (iter a result)
    (if (> a b)
	result
	(iter (next a) (+ result (term a)))))
  (iter a 0))

(define (pi-sum a b)
  (define (pi-term x)
    (/ 1.0 (* x (+ x 2))))
  (define (pi-next x)
    (+ x 4))
  (sum-iter pi-term a pi-next b))