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

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