def _start_transmit(self, stream):
        """
        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.
        """
        _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))
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.