(define (expt1 b n)
  (expt-iter b n 1))

(define (expt-iter b n a)
  (cond ((= n 0) a)
	((odd? n)
	       (expt-iter b (- n 1) (* a b)))
	(else (expt-iter (square b) (/ n 2) a))))

(define (square x) (* x x))