Exercise 1.9

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題目：

　　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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第一個函數：

　　inc (+ (dec a) b)，遞迴調用之後，還需要回到原函數進行inc計算，因此，是一個recursive process 

第二個函數：

　　尾遞迴，是一個iterative process

Exercise 1.9