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}