private bool NotUsed(PurchasedDevice purchasedDevice)
 => !_currentlyUsed.Any(usedDevice => purchasedDevice.IsSameDeviceAs(usedDevice));
 private UsedDevice GetUsedDeviceBy(PurchasedDevice device)
 {
     return(_currentlyUsed.First(usedDevice => device.IsSameDeviceAs(usedDevice)));
 }