m.respMu.Lock()
	defer m.respMu.Unlock()
	if m.closed {
		return errors.New("mux client closed")
	}
	return m.sendLocked(msg)
}

// sendLocked the message. msg.Seq and msg.MuxID will be set.
// m.respMu must be held.
func (m *muxClient) sendLocked(msg message) error {
	dst := GetByteBufferCap(msg.Msgsize())
	msg.Seq = m.SendSeq
	msg.MuxID = m.MuxID
	msg.Flags |= m.BaseFlags