code: func (ch *Channel) send(msg message) (err error) {
  	// If the channel is closed, use Channel.sendClosed()
  	if atomic.LoadInt32(&ch.closed) == 1 {
  		return ch.sendClosed(msg)
  	}
  
  	return ch.sendOpen(msg)
  }
docs: // send calls Channel.sendOpen() during normal operation.
  //
  // After the channel has been closed, send calls Channel.sendClosed(), ensuring
  // only 'channel.close' is sent to the server.