\begin{proposition}\label{phase5_formula_quantified_set_induction}
    $\forall x. x = x$.
\end{proposition}
\begin{proof}[Proof by \in-induction]
    Show $x = x$.
    \begin{subproof}
        Follows.
    \end{subproof}
    Follows by assumption.
\end{proof}
