$\forall$$T$:Type, ${\it as}$:($T$ List), $P$:(\{$a$:$T$$\mid$ ($a$ $\in$ ${\it as}$)\} $\rightarrow\mathbb{B}$). ($T$ $\subseteq$r $\mathbb{Z}$) $\Rightarrow$ (s{-}filter($P$;${\it as}$) $\in$ ($T$ List))