\begin{frame}
  \frametitle{Kripke Models}
  
  \begin{goal}{}
    A \emph{Kripke model} $\mathcal{M} = (W,R,\lab{L})$ consists of
    \begin{itemize}
      \item $W$, the \emph{worlds}
      \item $R$, the \emph{accessibility relation}
      \item $\lab{L}$, the \emph{labelling function}
    \end{itemize}
  \end{goal}

  \begin{exampleblock}{}
    \exampleA
  
    Formally:
    \begin{itemize}  
      \item  $W= \{\;w_1,\;w_2,\;w_3\;\}$
      \item  $R = \{ \; \pair{w_1}{w_2} ,\; \pair{w_1}{w_3},\; \pair{w_3}{w_2}\; \}$
      \item  $\lab{L}(w_1) = \{\; \lab{p} \;\}$  \quad
             $\lab{L}(w_2) = \{\; \lab{q} \;\}$  \quad
             $\lab{L}(w_3) = \{\; \lab{p,\; r} \;\}$
    \end{itemize}    
  \end{exampleblock}
\end{frame}