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

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