 A major methodological issue is then to strike a proper balance between expressive power and computational complexity, with the best multi-modal systems being those that manage to achieve a good compromise in this sense. We end this section noticing briefly that, in the case of the combination of modal logics, complexity depends deeply on the assumed bridge axioms. A famous example of this is the landmark paper by Halpern &amp; Vardi (1989) on the complexity of (96!) epistemic and temporal logics over interpreted systems, all of them differing on the used language (single or multiple-agents, common knowledge or lack of) and the assumed bridge principles (the aforementioned perfect recall and no learning, synchronicity, unique or multiple initial state).