Xeraph@NCHOVY

> (define (square x) (* x x))
> (define (smallest-divisor n)
    (find-divisor n 2))
> (define (find-divisor n test-divisor)
    (cond ((> (square test-divisor) n) n)
          ((divides? test-divisor n) test-divisor)
          (else (find-divisor n (+ test-divisor 1)))))
> (define (divides? a b)
    (= (remainder b a) 0))
> (define (prime? n)
    (= n (smallest-divisor n)))
> (prime? 19)
#t

연습문제 1.21

> (smallest-divisor 199)
199
> (smallest-divisor 1999)
1999
> (smallest-divisor 19999)
7

흠.. DrScheme에는 runtime이 없군.
current-milliseconds도 안 나오는데 이 어찌된 것인가 orz