$\forall$$T$:Type, $R$:($T$$\rightarrow$$T$$\rightarrow$Prop), $x$, $z$:$T$. ($x$ $R$\^{}+ $z$) $\Leftrightarrow$ ($\exists$$y$:$T$. ($x$ ($R$$^{\mbox{\scriptsize $\ast$}}$) $y$) \& ($y$ $R$ $z$))