no indices. A second advantage is that this collection can be iterated and thinned out at the same time. For example, when attacking with a transport ship which has some units loaded and <i>losing</i> the battle, this would remove several items from the collection. When this happens while being in an iteration through all units, most collection classes surely had a problem. ToughSet would handle this situation correctly, continue the iteration, not leave out units, not iterate units twice and not iterate un