\begin{frame}{Exercises (3)}
\begin{exampleblock}{}
Is there an NPDA $M$ with \alert{$L(M) = \{\, ww \mid w \in \{a,b \}^+ \,\}$}\,?
\end{exampleblock}
\pause\bigskip
\alert{No!}
\bigskip
This language is \alert{not context-free}.
\end{frame}
\themex{Acceptance with Empty Stack}