sicp1_16.scm

(define (fast-expt b n a)
  (cond ((= n 0) a)
        ((even? n) (fast-expt (* b b) (/ n 2) a))
        (else (fast-expt b (- n 1) (* a b)))))

(define (even? n)
  (= (remainder n 2) 0))

(define (expt b n)
  (fast-expt b n 1))