SICP习题1.33

(define (accumulate filter? combiner null-value term a next b)
  (cond ((> a b) null-value)
        ((filter? a) (combiner (term a)
                              (accumulate filter? combiner null-value term (next a) next b)))
        (else (accumulate filter? combiner null-value term (next a) next b))))

;以下部分是filter?的实现部分

(define (prime? n)
  (define (fermat-test times)
    (cond ((= times 0) true)
          ((try-it (random n)) (fermat-test (- times 1)))
          (else false)))
  (define (try-it a)
    (cond ((= (expmod a n) a) true)
          (else false)))
  (define (square x)
    (* x x)) 
  (define (expmod base exp)
    (cond ((= exp 0) 1)
          ((= (remainder exp 2) 0)
           (remainder (square (expmod base (/ exp 2))) n))
          (else
           (remainder (* base (remainder (expmod base (- exp 1)) n)) n))))
  (cond ((fermat-test 5) true)
        (else false)))

(define (gcd m n)
  (cond ((= n 0) m)
        (else (gcd n (remainder m n)))))