Let $X$ be a D11: Set such that
(i) | $\mathcal{P}(X)$ is the D80: Power set of $X$ |
(ii) | \begin{equation} \mathcal{X} : = \{ \{ x \} : x \in X \} \end{equation} |
Then
\begin{equation}
\mathcal{X}
\subseteq \mathcal{P}(X)
\end{equation}