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 ⟨??⟩ ⟨??⟩))