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\begin{frame}{Exercise}
  (Groups of two, 2 minutes)
  \begin{exampleblock}{}
    Construct a context-free grammar for the following NPDA.\\
    The stack starting symbol is $z=0$.
    \medskip

    \begin{center}
      \input{tikz/npda1.tex}
    \end{center}
    
    Use the grammar to show that $aabb$ is in the language.
  \end{exampleblock}
\end{frame}