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))