sicp exercise 1.30

;  Exercise 1.30. The sum procedure above generates a linear recursion. The procedure can be rewritten
;  so that the sum is performed iteratively. Show how to do this by filling in the missing expressions
;  in the following definition:
;  (define (sum term a next b)
;     (define (iter a result)
;         (if <??>
;             <??>
;             (iter <??> <??>)))
;     (iter <??> <??>))


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