number's power

(define (fast-expt b n)
    (define (even? a)
        (= (remainder a 2) 0))
    (define (check? a)
        (< (* a 2) n))
    (define (fast-expt-iter b count product)
        (cond ((< n count) product)
              ((and (even? count) (check? count)) (fast-expt-iter b (+ count count) (square product)))
              (else (fast-expt-iter b (+ count 1) (* b product)))))
(fast-expt-iter b 1 1))