1 ]=> ; Exercise 1.9.  Each of the following two procedures defines a method for adding two positive integers in terms of the procedures inc, which increments its argument by 1, and dec, which decrements its argument by 1.
(define (inc x)
  (+ x 1))
;Value: inc

1 ]=> (define (dec x)
  (- x 1))
;Value: dec

1 ]=> (define (+ a b)
  (display a)(newline)
  (if (= a 0)
      b
      (inc (+ (dec a) b))))
;Value: +

1 ]=> (+ 4 5)4
3
2
1
0
5
4
3
2
1
0
1
0
1
0
1
0
1
0
1
0