/// <summary>
/// deregisters the given unit UUID from UnitsInSight, updates VisionState accordingly
/// </summary>
/// <param name="UUID"></param>
public void UnitCannotSeeTile(string UUID)
{
if (UnitsInSight.Remove(UUID) && UnitsInSight.Count == 0)
{
UpdateVisibility(VisionState.SEEN);
}
}
