A D4: Binary relation $B = (X \times Y, R)$ is symmetric if and only if
\begin{equation}
\forall \, x \in X : \forall \, y \in Y
\left( (x, y) \in R \quad \implies \quad (y, x) \in R \right)
\end{equation}
A D4: Binary relation $B = (X \times Y, R)$ is symmetric if and only if
\begin{equation}
\forall \, x \in X : \forall \, y \in Y
\left( x R y \quad \implies \quad y R x \right)
\end{equation}
