code: func (o *ClientOptions) SetCleanSession(clean bool) *ClientOptions {
  	o.CleanSession = clean
  	return o
  }
docs: // SetCleanSession will set the "clean session" flag in the connect message
  // when this client connects to an MQTT broker. By setting this flag, you are
  // indicating that no messages saved by the broker for this client should be
  // delivered. Any messages that were going to be sent by this client before
  // diconnecting previously but didn't will not be sent upon connecting to the
  // broker.