$\forall$$T$:Type, $L$:($T$ List), $R$:(\{$x$:$T$$\mid$ ($x$ $\in$ $L$)\} $\rightarrow$\{$x$:$T$$\mid$ ($x$ $\in$ $L$)\} $\rightarrow\mathbb{P}$). sorted{-}by($R$;$L$) $\in$ $\mathbb{P}$