(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)
  ; a*b = x*y + z
  (define (iter x y z)
    ; ab = xy + z = x*0 = z
    (cond ((= y 0) z)
          ; ab = xy + z = (2x)(y/2) + z
          ((even? y) (iter (double x) (halve y) z))
          ; ab = xy + z = x(y - 1) + z + x
          (else (iter x (- y 1) (+ z x)))))
  (iter a b 0))