 However, there remain two necessitation rules, since there are still two boxes in \(C\). The resulting \({{\cal L}}\) is thus the fibring of \({{\cal L}}_1\) and \({{\cal L}}_2\) constrained by \(C_0\). This procedure precisely coincides with fusion of modal logics. The novelty here is that this technique applies to a broad class of logics, which are not necessarily restricted to (normal) modal logics, as in the case of fusion.