SICP 연습문제 풀이 1.18

(define (double a) (* a 2))
(define (halve a) (/ a 2))
(define (mult a b)
  (define (mult-iter a b c)
    (cond ((= b 0) c)
          ((even? b) (mult-iter (double a) (halve b) c))
          (else (mult-iter a (- b 1) (+ c a)))))
  (mult-iter a b 0))