Let $X$ be a D11: Set.
A D11: Set $\mathcal{C}$ is a cover for $X$ if and only if \begin{equation} X \subseteq \cup \mathcal{C} \end{equation}

Let $X$ be a D11: Set.
A D11: Set $\mathcal{C}$ is a cover for $X$ if and only if \begin{equation} X \subseteq \bigcup_{C \in \mathcal{C}} C \end{equation}