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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}