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Possible Worlds/ Actuality Operator / Simons: with an a A.O. we can avoid reference to possible worlds - e.g. if there is a (non-empty) set in a world and all of its elements also exist in other world, then there is the set itself in that world - then without possible worlds: N (a) N1 (P (Ea u (x) [x e a > A1 (e!x)] > Ea) - that is, when a part of b is in a world, then also in each world, in which b exists.
Parts Oxford New York 1987