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\begin{frame}{Looking Back}
  \structure{Previous subjects relevant for this lecture:}
  \begin{itemize}\setlength{\itemsep}{2ex}
    \medskip
    \item languages as sets of words
    \item finite automata: deterministic and non-deterministic
    \item language $L$ is \alert{regular} $\iff$ $L$ accepted by a \alert{DFA}
    \item language $L$ is \alert{regular} $\iff$ $L$ accepted by a \alert{NFA}
  \end{itemize} 
\end{frame}