\begin{frame}
  \frametitle{Special Cases}
  
  \begin{exampleblock}{}
  \begin{tikzpicture}
  \naturaldeduction{
    \proofbox{
      \proofstep{$p$}{assumption};
    }
    \proofstep{$p \to p$}{$\to_i$ 1--1};
  }
  \end{tikzpicture}
  \medskip
  
  This is a derivation of 
  \begin{talign}
    \vdash\; p \to p
  \end{talign}
  \end{exampleblock}
\end{frame}