code: async def _retransmit(self, sequence_number):
          
          packet = self.__rtp_history.get(sequence_number % RTP_HISTORY_SIZE)
          if packet and packet.sequence_number == sequence_number:
              if self.__rtx_payload_type is not None:
                  packet = wrap_rtx(packet,
                                    payload_type=self.__rtx_payload_type,
                                    sequence_number=self.__rtx_sequence_number,
                                    ssrc=self._rtx_ssrc)
                  self.__rtx_sequence_number = uint16_add(self.__rtx_sequence_number, 1)
  
              self.__log_debug('> %s', packet)
              packet_bytes = packet.serialize(self.__rtp_header_extensions_map)
              await self.transport._send_rtp(packet_bytes)
docs: Retransmit an RTP packet which was reported as lost.