Let $X$ be a D11: Set such that

(i)	\begin{equation} X \neq \emptyset \end{equation}
(ii)	${\sim} \subseteq X \times X$ is an D178: Equivalence relation on $X$

The equivalence class of $x \in X$ with respect to ${\sim}$ is the D11: Set \begin{equation} \{ y : (x, y) \in {\sim} \} \end{equation}