\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}