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