Let $\emptyset$ be the D13: Empty set.
Let $X_1, \ldots, X_{N + 1}$ each be a D11: Set such that

(i)	\begin{equation} X_{N + 1} = \emptyset \end{equation}

Then \begin{equation} \bigcup_{n = 1}^{N + 1} X_n = \bigcup_{n = 1}^N X_n \end{equation}