费马小定理（确定n 是否为素数）

(define (expmod base exp m )
(cond ((=exp 0 ) 1)
((even? exp) (remainder (square (expmod base (/ exp 2) m)) m))
(else (remainder (* base (expmod base (- exp 1) m)) m))
))
(define (fermat-test n)
(define (try-it a) (= (expmod a n n) a))
(try-it (+1 (random (- n 1)))))
(define (fast-prime? n times)
(cond ((= times 0) true)
((fermat-test n) (fast-prime? n (- times 1)))
(else false) ))
(define (even? n)
(= (remainder n 2) 0))