Let $X$ be a D11: Set.
Let $E_n$ be a D11: Set for each $n \in \mathbb{N}$ such that

(i)	$\bigcup_{n \in \mathbb{N}} E_n$ is the D77: Set union of $E = \{ E_n \}_{n \in \mathbb{N}}$

Then \begin{equation} \bigcup_{n \in \mathbb{N}} E_n \subseteq X \quad \iff \quad \forall \, n \in \mathbb{N} : E_n \subseteq X \end{equation}