 Theorem (Baltag, Moss, and Solecki 1998, 1999; see also van Ditmarsch, van der Hoek, and Kooi 2007). Over the class of all pointed Kripke models, the language \eqref{PAL+C} of public announcement logic with common knowledge is strictly more expressive than language \eqref{PAL} without common knowledge. In particular, the \eqref{PAL+C}-formula \([p!][C]q\) cannot be expressed in \eqref{PAL} with respect to the class of all pointed Kripke models: for every \eqref{PAL}-formula F there exists a pointed Kripke model \((M,w)\) such that \(M,w\not\models F\leftrightarrow[p!][C]q\).