\begin{prooftree}
\AxiomC{$A \seq A$}
\RightLabel{\scriptsize $\lnot\ \text{R}$}
\UnaryInfC{$\seq A, \lnot A$}
\AxiomC{$A \seq A$}
\RightLabel{\scriptsize $\limplies\ \text{L}$}
\BinaryInfC{\lnot A \limplies A \seq A}
\RightLabel{\scriptsize $\limplies\ \text{R}$}
\UnaryInfC{$(\lnot A \limplies A) \seq A$}
\end{prooftree}