 For a deeper study on the fusion of modal logics, the reader is referred to Wolter (1998), Gabbay, Kurucz, Wolter, and Zakharyaschev (2003: Chapter 4), and Kurucz (2006: Section 2). Further examples of fusion can be found  in the SEP entry  discussing methods for  combining logics.  Still, before closing this subsection, we add a word of caution. One needs to be careful when building the fusion of modal systems. This is because, in the system that results from the fusion, there are already formulas combining the modalities of its different fragments. Then, even if no particular bridge (valid) principles are enforced, the language might gain in expressive power, which might increase its  complexity  profile.  Section 3.4  on complexity issues elaborates on this important but often forgotten aspect.