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

  \begin{example}{}
  \begin{center}
    \begin{tikzpicture}[
        default,
        point/.style={circle, draw=blue, thick, inner sep=3pt, minimum size=9mm},
        node distance=20mm]
  
      \node (1) [point] {\lab{p}};
      \node [alo=1] {$w_1$};
      
      \node (2) [point, right of=1] {\lab{q}};
      \node [aro=2] {$w_2$};

      \node (3) [point, below of=1] {\lab{p,\;r}};
      \node [alo=3] {$w_3$};

      \node (4) [point, right of=3] {\lab{r}};
      \node [aro=4] {$w_4$};

      \begin{scope}[shorten <= 1mm, shorten >= 1mm, very thick,>=stealth]
        \draw [->] (1) to[bend left=15] (2);
        \draw [->] (2) to[bend left=15] (1);
        \draw [->] (1) to (3);
        \draw [->] (3) to (2);
        \draw [->] (2) to (4);
      \end{scope}
    \end{tikzpicture}
    \vspace{1ex}
  \end{center}
  
  Determine the truth value of every formula in every world:
  \begin{talign}
    &\some\all q & \mbox{\DARKRED{?}} \\
    &\some\some\all q & \mbox{\DARKRED{?}} \\
    &\all\some\all (q \vee r) & \mbox{\DARKRED{?}} \\
    &\some(\all (q \vee r) \to p)  & \mbox{\DARKRED{?}} \\
    &\all(\some p \to \some\some r)  & \mbox{\DARKRED{?}}
  \end{talign}
  \end{example}
\end{frame}