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

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

Using the substitution model, illustrate the process generated by each
procedure in evaluating (+ 4 5).  Are these processes iterative or
recursive?