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\begin{frame}{Exercise}
  Let
  \begin{itemize}
    \item $\Sigma = \{\,a,b\,\}$
    \item $L_1 = \{\, a,\; bb \,\}$
    \item $L_2 = \{\, ab,\; ba \,\}$
  \end{itemize}  
  \bigskip
  
  (In groups of two, 2 minutes)
  \begin{exampleblock}{}
  \emph{Describe the following languages as sets:}
  \begin{itemize}
    \item $L_1 L_2 = \mpause[1]{\{\,aab,\; aba,\; bbab,\; bbba\,\}}$
    \item $L_1^2 = \mpause{\{\, aa,\; abb,\; bba,\; bbbb\,\}}$
    \item $L_1^3 = \mpause{\text{answered in the lecture}}$
  \end{itemize}
  \end{exampleblock}
\end{frame}