code: func (s DeviceSession) GetDownlinkGatewayMAC() (lorawan.EUI64, error) {
  	for mac := range s.UplinkGatewayHistory {
  		return mac, nil
  	}
  
  	return lorawan.EUI64{}, errors.New("uplink gateway-history is empty")
  }
docs: // GetDownlinkGatewayMAC returns the gateway MAC of the gateway close to the
  // device.
  // TODO: refactor so that it uses GetDeviceGatewayRXInfoSet.