 More expressive languages in epistemic logic involve the addition of operators for various notions of group knowledge (see  Section 3).  For example, as we discuss below, the common knowledge operator and so-called dynamic operators are important additions to the language of epistemic logic. Dynamic operators can indicate for example the truthful public announcement of \(\varphi\): \([\varphi!]\). A formula \([\varphi!]\psi\) is read “if \(\varphi\) is truthfully announced to everybody, then after the announcement, \(\psi\) is the case”. The question of what kinds of expressive power is added with the addition of operators is a research topic that is actively being investigated in  dynamic epistemic logic.  So, for example, adding \([\varphi!]\) by itself to \(\mathcal{L}_{KB}\) does not add expressive power, but in a language that also includes common knowledge, it does.