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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}