SICP 练习1.17

(define (double x) (+ x x))(define (halve x) (/ x 2))(define (even? x)  (= (remainder x 2) 0))(define (multiply a b)  (cond ((= b 0) 0)        ((even? b) (double (multiply a (halve b))))        (else (+ a (multiply a (- b 1))))))(multiply 7 9)