(define (filtered-accumulate combiner null-value term a next b filter?)
  (define (iter a result)
    (if (> a b)
        result
        (iter (next a) (if (filter? a)
                           (combiner (term a) result)
                           result))))
  (iter a null-value))

; (define (filtered-accumulate combiner null-value term a next b filter?)
;   (if (> a b)
;       null-value
;       (if (filter? a)
;           (combiner (term a)
;                     (filtered-accumulate combiner null-value term (next a) next b filter?))
;           (filtered-accumulate combiner null-value term (next a) next b filter?))))

(define (sum a b) (filtered-accumulate + 0 identity a inc b identity))
(define (prod a b) (filtered-accumulate * 1 identity a inc b identity))

(define (prime-square-sum a b) (filtered-accumulate + 0 square a inc b prime?))

(define (b-prime-product b)
  (define (is-prime-for-b x) (= (gcd b x) 1))
  (filtered-accumulate * 1 identity 1 inc b is-prime-for-b))

(define (smallest-divisor n)
  (define (find-divisor test-divisor)
    (cond ((> (square test-divisor) n) n)
          ((divides? test-divisor n) test-divisor)
          (else (find-divisor (+ test-divisor 1)))))
  (find-divisor 2))

(define (divides? a b)
  (= (remainder b a) 0))

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

(define (gcd a b)
  (if (= b 0)
    a
    (gcd b (remainder a b))))