A D1106: Topological space $T = (X, \mathcal{T})$ is a separable topological space if and only if there exists a D11: Set $S \subseteq X$ such that

(1)	\begin{equation} \forall \, U \in \mathcal{T} \left( U \neq \emptyset \quad \implies \quad U \cap E \neq \emptyset \right) \end{equation}
(2)	$S$ is a D16: Countable set