1.16.scm

(define (square x) (* x x))
(define (even? n) (= (remainder n 2) 0))

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