#lang sicp

(define (double x) (* 2 x))
(define (halve x) (/ x 2))
(define (*aux a b acc)
  (cond ((= b 0) acc)
        ((even? b) (*aux (double a) (halve b) acc))
        (else      (*aux a (- b 1) (+ acc a)))))
(define (fast-mul a b)
  (*aux a b 0))