code: private boolean checkNlTaxNumber(final String ptaxNumber) {
      final int checkSum = ptaxNumber.charAt(8) - '0';
      int sum = 0;
      for (int i = 0; i < ptaxNumber.length() - 1; i++) {
        sum += (ptaxNumber.charAt(i) - '0') * (9 - i);
      }
      sum -= checkSum;
      return sum % 11 == 0;
    }
docs: check the Tax Identification Number number, country version for Netherlands.
  
  @param ptaxNumber vat id to check
  @return true if checksum is ok