when $P$, $R_{1}$ =$>$ $R_{2}$ $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$x$, $y$:$T$. $P$($x$) $\Rightarrow$ ($x$ $R_{1}$ $y$) $\Rightarrow$ ($x$ $R_{2}$ $y$)