code: def _start_transmit(self, stream):
          
          _vv and IOLOG.debug('%r._start_transmit(%r)', self, stream)
          side = stream.transmit_side
          assert side and side.fd is not None
          self.poller.start_transmit(side.fd, (side, stream.on_transmit))
docs: Mark the :attr:`transmit_side <Stream.transmit_side>` on `stream` as
          ready for writing. Must only be called from the Broker thread. When the
          associated file descriptor becomes ready for writing,
          :meth:`BasicStream.on_transmit` will be called.