 In possibilistic versions of situation semantics, possible situations are parts of possible worlds. Some authors also assume that the parts of a possible world w form a join semi-lattice with maximal element w (Bach 1986; Lasersohn 1988, 1990; Portner 1992; see also the entry  mereology).  The part relation ≤p and the sum operation + are then related as usual: s ≤p s′ iff s + s′ = s′. Propositions are sets of possible situations or their characteristic functions (see the entry  propositions).  The notion of a situation that exemplifies a proposition might be defined as in (29), which is a variation of a definition that appears in Kratzer 1990 (Other Internet Resources), 1998, 2002: