$\forall$$T$:Type, $l$:($T$ List), $P$:(\{$x$:$T$$\mid$ ($x$ $\in$ $l$)\} $\rightarrow\mathbb{B}$). filter($P$;$l$) $\in$ (\{$x$:$T$$\mid$ ($x$ $\in$ $l$)\}  List)