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 (plus1 a b)
(if (= a 0)
b
(inc (plus1 (dec a) b))))
(define (plus2 a b)
(if (= a 0)
b
(plus2 (dec a) (inc b))))
Using the substitution model, illustrate the process generated by each procedure in evaluating (+ 2 5). Are these processes iterative or recursive?