1  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:
2 
3 (define (sum term a next b)
4   (define (iter a result)
5     (if <??>
6         <??>
7         (iter <??> <??>)))
8   (iter <??> <??>))