Ex-1.18

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

;to find multiplication of a and b, call
; (mul-iter 0 a b)
(define (mul-iter r a b)
  (cond ((= b 0) r)
        ((even? b)
         (mul-iter r (double a) (halve b)))
        (else
         (mul-iter (+ r a) a (- b 1)))))