Consider the following NPDA $M$:
      \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);
    What is the language accepted by this automaton?
      L(M) = \mpause[1]{ \{\, a^n b^m \mid n \ge m \ge 1 \,\} }
    The empty stack language of $M$ is 
      L_\lambda(M) = \mpause[1]{ \{\, a^n b^n \mid n \ge 1 \,\} }
    For a language $L$ the following two are equivalent:
      \item There is an NDPA $M$ with $L(M) = L$.
      \item There is an NDPA $M$ with $L_\lambda(M) = L$.