“…This method often results in fragments which have a good compromise between computational complexity and expressive power. Such fragments have been studied, e.g., in the case of linear temporal logic [1], temporal description logics [2], interval temporal logics [5,4], and, recently, normal modal logics K, T, K4, S4, and S5 [15,6]. Recall that K is the basic modal logic which semantically corresponds to the class of relational structures with an arbitrary binary relation, whereas in T, K4, S4, and S5 the relation is reflexive, transitive, a preorder (i.e., reflexive and transitive), and an equivalence, respectively.…”