ex1.9-substitute-mode-eval-incre-decre.scm

; Ex1.9
; (define (+ a b)
;   (if (= a 0)
;       b
;       (inc (+ (dec a) b))))

; (define (+ a b)
;   (if (= a 0)
;       b
;       (+ (dec a) (inc b))))
; (+ 4 5)

; 1st:
; (+ 4 5)
; (inc (+ 3 5))
; (inc (inc (+ 2 5)))
; (inc (inc (inc (+ 1 5))))
; (inc (inc (inc (inc (+ 0 5)))))
; (inc (inc (inc (inc 5))))
; (inc (inc (inc 6)))
; (inc (inc 7))
; (inc 8)
; 9