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

; multiply a by positive integer b
(define (mult a b)
  (cond ((= b 1) a)
        ((even? b) (mult (double a) (halve b)))
        (else (+ a (mult a (- b 1))))))