\begin{frame}{Theorem of Rice: Example}
  \begin{exampleblock}{}
    For recursively enumerable languages $L$, the following questions are undecidable:
    \begin{enumerate}
      \item Is $a\in L$?
      \item Is $L$ finite?
    \end{enumerate}
  \end{exampleblock}
\end{frame}

\themex{Post Correspondence Problem}