“…For dealing with modal clauses, a set of hyper-resolution rules [44] are applied to modal and propositional clauses. The main difference between those proof methods resides in their normal form: in [33], for completeness, all clauses are considered for application of the resolution rules; whilst in [34], because clauses are labelled, the resolution rules only need to be applied when the labels of clauses can be unified. Differently from other calculi which use labels for guiding the application of inference rules [3,5,10,11,54,55], the labels in the clausal form given in [34] do not refer to worlds, but to the modal layer (i.e.…”