 Fusion, even if it is a very natural method for combining modal logics, however, is not obviously extendable to combinations of non-normal modal logics with normal modal logics. Moreover, fusion is specifically designed for combining modal logics, and cannot be extended in an obvious way to logics of a different nature. Algebraic fibring, described in  subsection 4.3  below, constitutes a generalization of fusion (at the propositional level), and generally solves the question of combining logics.