one{-}one($A$;$B$;$R$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$x$:$A$. $\forall$$y$:$B$, $z$:$B$. ($R$($x$,$y$)) $\Rightarrow$ ($R$($x$,$z$)) $\Rightarrow$ ($y$ = $z$ $\in$ $B$)