code: def _add_check(self, node, run, language, is_code_node):
          
          def run_check():
              """Yield errors."""
              all_results = run()
              if all_results is not None:
                  if all_results:
                      for result in all_results:
                          error_offset = result[0] - 1
  
                          line_number = getattr(node, 'line', None)
                          if line_number is not None:
                              yield (
                                  beginning_of_code_block(
                                      node=node,
                                      line_number=line_number,
                                      full_contents=self.contents,
                                      is_code_node=is_code_node) +
                                  error_offset,
                                  '({}) {}'.format(language, result[1]))
                  else:
                      yield (self.filename, 0, 'unknown error')
          self.checkers.append(run_check)
docs: Add checker that will be run.