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\begin{frame}{Pushdown Automata}
\begin{block}{}
A \emph{nondeterministic pushdown automaton} (\emph{NPDA}) is a tuple
\begin{talign}
M = (Q,\Sigma,\Gamma,\delta,q_0,z,F)
\end{talign}
\begin{itemize}
\item $Q$ is a finite set of states
\item $\Sigma$ is a finite input alphabet
\item \alert{$\Gamma$} is a finite stack alphabet
\item \alert{$\delta : Q \times (\Sigma \cup \{\, \lambda \,\}) \times \alert{\Gamma} \to 2^{Q \times \alert{\Gamma^*}}$}

the transition function, where $\delta(q,\alpha,b)$ is always finite
\item $q_0 \in Q$ the starting state
\item \alert{$\alert{z} \in \Gamma$} the stack starting symbol
\item $F \subseteq Q$ a set of final states
\end{itemize}
\end{block}
\end{frame}