Ex-1.16

;to calculate (expt b n) call (fast-expt-iter 1 b n)
(define (fast-expt-iter a b n)
  (cond ((= n 0) a)
        ((even? n)
         (fast-expt-iter a (square b) (/ n 2)))
        (else (fast-expt-iter (* a b) b (- n 1)))))
(define (even? n)
  (= (remainder n 2) 0))