A D4424: Binary endorelation $B = (X \times X, R)$ is reflexive if and only if \begin{equation} \forall \, x \in X : (x, x) \in R \end{equation}

A D4424: Binary endorelation $B = (X \times X, R)$ is reflexive if and only if \begin{equation} \{ (x, x) : x \in X \} \subseteq R \end{equation}