$\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