\begin{frame}
  \frametitle{Examples}

  \begin{exampleblock}{$R(a,a) \;\vdash\; \myex{x}{\alert{R(x,x)}}$}
  \begin{tikzpicture}
    \naturaldeduction{
      \proofstep{$R(a,a)$}{premise};   
        \proofstep{$\myex{x}{R(x,x)}$}{$\exists i\;$ 1};
    }
  \end{tikzpicture}
  \end{exampleblock}
  \pause\medskip
  
  \begin{exampleblock}{$\vdash\; R(a,a) \alert{\to} \myex{x}{R(x,x)}$}
  \begin{tikzpicture}
    \naturaldeduction{
    \proofbox{
      \proofstep{$R(a,a)$}{assumption};   
        \proofstep{$\myex{x}{R(x,x)}$}{$\exists i\;$ 1};
    }
    \proofstep{$R(a,a) \to \myex{x}{R(x,x)}$}{$\to i\;$ 1--2};
    }
  \end{tikzpicture}
  \end{exampleblock}
\end{frame}