Ex-1.17

(define (double x) (* 2 x))
(define (halve x) (/ x 2))
(define (even? n)
  (= (remainder n 2) 0))

(define (mul a b)
  (cond
   ((= b 1) a)
   ((even? b)
    (double (mul a (halve b))))
   (else (+ a (mul a (- b 1))))))