\begin{frame}{Pushdown Automata}
  \begin{goal}{Goal}
    A class of automata that accepts the context-free languages.
  \end{goal}
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  Nondeterministic finite automata (NFA's):
  \begin{itemize}\setlength{\itemsep}{0pt}
    \item no memory except for the current state
    \item has only finitely many states
  \end{itemize}
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  \begin{exampleblock}{}
    We need some form of infinite memory to accept languages like
    \begin{talign}
      \{\,a^nb^n \mid n \ge 0 \,\}
    \end{talign}
  \end{exampleblock}  
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  \begin{goal}{Pushdown automataon}
    A pushdown automaton has a \emph{stack} of unlimited size. 
  \end{goal}  
\end{frame}