 A model for this language is a structure \(M = ((W, R_1, \ldots, R_n, \ldots), V)\), consisting of a set of worlds or states or outcomes \(W\); an accessibility relation \(R_i\) for each modal operator \(\Box_i\), defined via so-called neighborhood functions (Chellas 1980), i.e., functions \(R_i: W \to 2^{2^{W}}\); and a valuation function \(V: \texttt{Atoms} \to 2^{W}\), which assigns to each atomic proposition a subset of \(W\), with the idea that each atomic proposition is assigned to the set of worlds at which this proposition is true.