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\begin{frame}{Variations of Turing Machines}
  \begin{goal}{}
  We consider the following variations of Turing machines:
  \begin{itemize}
    \item Turing machines with multiple tapes 
    \item nondeterministic Turing machines
    \item universal Turing machines 
  \end{itemize}
  \end{goal}
\end{frame}