\let\erwin=\natural
\let\miezi=\cat  \let\miezi=\active
$\erwin\owns\miezi$

\setbox0=\vbox{\the\miezi}
\bigbreak

\box0

\ifcat\miezi\active
    \relax
\else
    \hang\it!
    \phantom{\the\miezi}
\fi
\end