  motivated purely by greed, members of group Q descend on the water hole possessed by group P and insist on sharing that with them. (What’s more they do not allow reciprocity; they do not allow members of P to share any water hole that was legitimately in the possession of Q.) That is an injustice. But then circumstances change, and all the water holes of the territory dry up except the one that originally belonged to P. The members of group Q are already sharing that water hole on the basis of their earlier incursion. But now that circumstances have changed, they are entitled to share the water hole; so that their doing so no longer counts as an injustice. It is in fact part of what justice requires. The initial injustice of Q against P has been superseded by circumstances. (Waldron 1992: 25)