the demonstration of nonlocality due to bell is probabilistic in the sense that it shows that the precise probabilities predicted by quantum mechanics for some entangled scenarios cannot be met by a local theory. (for short, here and henceforth "local theory" means "local hidden variables theory".) however, quantum mechanics permits an even stronger violation of local theories: a possibilistic one, in which local theories cannot even agree with quantum mechanics on which events are possible or impossible in an entangled scenario. the first proof of this kind was due to greenberger, horne and zeilinger in 1993