(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 (square x)
   (* x x))

(define (prime? n)
   (= n (smallest-divisor n)))

(time
  (display (prime? 10000000019))
  (newline))

#t
cpu time: 40 real time: 288 gc time: 10