code: def _verify_client_authentication(self, request_body, http_headers=None):
          # type (str, Optional[Mapping[str, str]] -> Mapping[str, str]
          
          if http_headers is None:
              http_headers = {}
          token_request = dict(parse_qsl(request_body))
          token_request['client_id']  = verify_client_authentication(self.clients, token_request, http_headers.get('Authorization'))
          return token_request
docs: Verifies the client authentication.
          :param request_body: urlencoded token request
          :param http_headers:
          :return: The parsed request body.