preserved\_by2($T$;$R$;$P$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$x$:$T$. $\forall$$y$:$T$, $z$:$T$. $P$($x$) $\Rightarrow$ $P$($y$) $\Rightarrow$ $R$($x$,$y$,$z$) $\Rightarrow$ $P$($z$)