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

; a.
; express the sum of squares of primes between a and b using filter-accum

(define (sum-of-squares-of-primes-between a b)
  (filtered-accumulate + 0 square a prime? inc b))

; b.
; express the sum of positive integers less than, and relatively prime to, n

(define (relatively-prime? m n) (= (gcd m n) 1))

(define (id x) x)

(define (sum-of-relatively-prime-predecessors n)
  (define (rel-prime-to-n? m) (relatively-prime? m n))
  (filtered-accumulate + 0 id 1 rel-prime-to-n? inc b))