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\begin{frame}{Looking Back}
  \structure{Previous subjects relevant for this lecture:}
  \medskip
  \begin{itemize}\setlength{\itemsep}{2ex}
    \item Context-free languages via context-free grammars
    \item Context-free languages via pushdown automata
  \end{itemize}
  \pause\bigskip

  \begin{question}
    Are all languages context-free?
  \end{question}
\end{frame}