 This new modality \([&lt;]\) allows us to define even more epistemic notions. For example, a formula \(\varphi\) is weakly safely believed (a belief which might be lost but is never reversed when revising with true information) if and only if \(\varphi \land [{&lt;}] \varphi\) holds. More details can be found in Baltag and Smets (2008: Subsection 2.4).