**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 <??> <??>))
```

The iterative solution:

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