#lang scheme

(define (power b n)
  (define (square x)
    (* x x))
  (define (even? n)
    (= (remainder n 2) 0))
  (define (power-iter a b n)
    (cond ((and (= b 0) (= n 0)) "undefined")
          ((= b 0) 0)
          ((= n 0) a)
          ((even? n) (power-iter a
                                 (square b)
                                 (/ n 2)))
          (else (power-iter (* a b)
                            b
                            (- n 1)))))
    (power-iter 1 b n))