SICP Exercise 1.9.

A.

(define (+ a b)
  (if (= a 0)
      b
      (inc (+ (dec a) b))))

B.

(define (+ a b)
  (if (= a 0)
      b
      (+ (dec a) (inc b))))

A.

(+ 4 5)
(inc (+ (dec 4) 5))
(inc (inc (+ (dec 3) 5))))
(inc (inc (inc (+ (dec 2) 5))))
(inc (inc (inc (inc (+ (dec 1) 5)))))
(inc (inc (inc (inc 5))))
(inc (inc (inc 6)))
(inc (inc 7))
(inc 8)
9

This is a recursive process.

B.

(+ 4 5)
(+ (dec 4) (inc 5))
(+ (dec 3) (inc 6))
(+ (dec 2) (inc 7))
(+ (dec 1) (inc 8))
9

This is a iterative process.