code: private boolean isTaxDocument(String argument) {
          try {
              Integer.parseInt(argument.substring(0,1));
          } catch (Exception ex) {
              return false;
          }
          return true;
      }
docs: This method is used to validate the argument of bellow method, if probably it's a tax document or not.
  
  @param   argument
  {@code String} the received argument.
  
  @return  {@code boolean}