User blog comment:Deedlit11/Ordinal Notations V: Up to a weakly Mahlo cardinal/@comment-30004975-20180124232836/@comment-5529393-20180130085437

Normally, we only take \(\psi_\pi\) when \(\pi\) is a regular cardinal; if we apply the definition anyway, we get \(\psi_{\Omega_\Omega}(0) = \Omega_{\Gamma_0}\).