Let $X$ be a D11: Set such that

(i)	\begin{equation} X \neq \emptyset \end{equation}
(ii)	$x \in X$ is a D2218: Set element in $X$
(iii)	${\sim} \subseteq X \times X$ is an D178: Equivalence relation on $X$

Then \begin{equation} x \in \{ y : (x, y) \in {\sim} \} \end{equation}