Exercise 1.9

题目：

　　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?

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第一个函数：

　　inc (+ (dec a) b)，递归调用之后，还需要回到原函数进行inc计算，因此，是一个recursive process 

第二个函数：

　　尾递归，是一个iterative process