\begin{definition}\label{phase5_finite_set_definition}
    $\phasefivefiniteset{a}{b} = \{ a, b \}$.
\end{definition}

\begin{proposition}\label{phase5_finite_set_theorem}
    For all $a, b$ we have $a \in \{ a, b \}$.
\end{proposition}
\begin{proof}
    Fix $a, b$.
    Follows by assumption.
\end{proof}