(define make-verifier
  (lambda (process modulus)
    (define combine-the-digits ; still needs some work
      (lambda (n)
        (define sum-plus ;(sum of processed versions of n's digits) + addend
          (lambda (n addend position)
            (if (= n 0)
                addend
                (sum-plus (quotient n 10)
                          (+ addend 
                             (process position
                                      (remainder n 10)))
                          (+ position 1)))))
        (sum-plus n 0 1)))
    (lambda (number)
      (= (remainder (combine-the-digits number) modulus) 0))))

(define isbn-verifier (make-verifier * 11))

(define upc-verifier (make-verifier (lambda (pos dig)
                                      (if (odd? pos)
                                          dig
                                          (* 3 dig)))
                                    10))