rel\_equivalent($T$;$R_{1}$;$R_{2}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$x$:$T$, $y$:$T$. ($R_{1}$($x$,$y$)) $\Leftarrow\!\Rightarrow$ ($R_{2}$($x$,$y$))