\begin{frame}
  \frametitle{Exam Preparation Exercises}

  \begin{example}{}
  \begin{center}
    \begin{tikzpicture}[
        default,
        point/.style={circle, draw=blue, thick, inner sep=3pt, minimum size=7mm},
        node distance=20mm]
  
      \node (1) [point] {};
      \node [ro=1] {$w_1$};

      \node (2) [point,below left of=1,xshift=-10mm] {};
      \node [ro=2] {$w_2$};

      \node (3) [point,below right of=1,xshift=10mm] {};
      \node [ro=3] {$w_3$};

      \node (4) [point,below left of=2] {};
      \node [ro=4] {$w_4$};
      
      \node (5) [point,below right of=2] {};
      \node [ro=5] {$w_5$};

      \node (6) [point,below of=3] {};
      \node [ro=6] {$w_6$};

      \node (7) [point,below left of=6] {};
      \node [ro=7] {$w_7$};
      
      \node (8) [point,below right of=6] {};
      \node [ro=8] {$w_8$};

      \begin{scope}[shorten <= 1mm, shorten >= 1mm, very thick,>=stealth]
        \draw [->] (1) -- (2);
        \draw [->] (1) -- (3);
        \draw [->] (2) -- (4);
        \draw [->] (2) -- (5);
        \draw [->] (3) -- (6);
        \draw [->] (6) -- (7);
        \draw [->] (6) -- (8);
      \end{scope}
      
      \node at (7) {$\lab{p}$};
      \node at (8) {$\lab{p}$};
      \node at (5) {$\lab{p}$};
      \node at (2) {$\lab{p}$};
    \end{tikzpicture}
    \vspace{1ex}
  \end{center}
  
  Determine in which worlds the following formula holds:
  \begin{talign}
    &\all\some\all\some p  & \mbox{\DARKRED{?}}
  \end{talign}
  \end{example}
\end{frame}