def _send_frame(self, connection, frame):
        """
        Sends a frame to a specific subscriber connection.

        (This method assumes it is being called from within a lock-guarded public
        method.)

        @param connection: The subscriber connection object to send to.
        @type connection: L{coilmq.server.StompConnection}

        @param frame: The frame to send.
        @type frame: L{stompclient.frame.Frame}
        """
        assert connection is not None
        assert frame is not None

        self.log.debug("Delivering frame %s to connection %s" %
                       (frame, connection))

        if connection.reliable_subscriber:
            if connection in self._pending:
                raise RuntimeError("Connection already has a pending frame.")
            self.log.debug(
                "Tracking frame %s as pending for connection %s" % (frame, connection))
            self._pending[connection] = frame

        connection.send_frame(frame)
Sends a frame to a specific subscriber connection.

        (This method assumes it is being called from within a lock-guarded public
        method.)

        @param connection: The subscriber connection object to send to.
        @type connection: L{coilmq.server.StompConnection}

        @param frame: The frame to send.
        @type frame: L{stompclient.frame.Frame}