\begin{frame}{Example}
  \begin{exampleblock}{}
    Consider the following NPDA $M$:
    \begin{center}
      \vspace{-1ex}
      \begin{tikzpicture}[default,node distance=25mm,->,s/.style={minimum size=5mm}]
        \node (q0) [state,s] {$q_0$}; \draw ($(q0) + (-8mm,0mm)$) -- (q0); 
        \draw (q0) to[tloop] node [label,above] {$a[z/1]$} (q0);
        \draw (q0) to[bloop] node [label,below] {$a[1/11]$} (q0);

        \node (q1) [fstate,s,right of=q0] {$q_1$};
        \draw (q0) to node [label,above] {$b[1/\lambda]$} (q1);
        \draw (q1) to[tloop] node [label,above] {$b[1/\lambda]$} (q1);
      \end{tikzpicture}
      \vspace{-2ex}
    \end{center}
    What is the language accepted by this automaton?
    \begin{talign}
      L(M) = \mpause[1]{ \{\, a^n b^m \mid n \ge m \ge 1 \,\} }
    \end{talign}
    \pause\pause
    The empty stack language of $M$ is 
    \begin{talign}
      L_\lambda(M) = \mpause[1]{ \{\, a^n b^n \mid n \ge 1 \,\} }
    \end{talign}
  \end{exampleblock}
  
  \pause\pause
  
  \begin{goal}{}
    For a language $L$ the following two are equivalent:
    \begin{itemize}
      \item There is an NDPA $M$ with $L(M) = L$.
      \item There is an NDPA $M$ with $L_\lambda(M) = L$.
    \end{itemize}
  \end{goal}
\end{frame}