Exercise 1.22

(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)))

(define (timed-prime-test p? n)
  (newline)
  (display n)
  (start-prime-test p? n (runtime)))
(define (start-prime-test p? n start-time)
  (when (p? n)
    (report-prime (- (runtime) start-time))))
(define (report-prime elapsed-time)
  (display " *** ")
  (display elapsed-time))

(define (search-for-primes p? a b)
  (define (iter a b)
    (when (<= a b)
      (timed-prime-test p? a)
      (iter (+ a 2) b)))
  (iter (if (odd? a) a (+ a 1)) b))

(string-contains?
 (capture-output (search-for-primes prime? 6 10))
 "7 *** ")
=> #t