code: func (o *ClientOptions) SetProtocolVersion(pv uint) *ClientOptions {
  	if (pv >= 3 && pv <= 4) || (pv > 0x80) {
  		o.ProtocolVersion = pv
  		o.protocolVersionExplicit = true
  	}
  	return o
  }
docs: // SetProtocolVersion sets the MQTT version to be used to connect to the
  // broker. Legitimate values are currently 3 - MQTT 3.1 or 4 - MQTT 3.1.1