$\forall$$T$:Type, $P$:($T$$\rightarrow\mathbb{B}$), $L_{2}$, $L_{1}$:($T$ List). $L_{1}$ $\leq$ $L_{2}$ $\Rightarrow$ filter($P$;$L_{1}$) $\leq$ filter($P$;$L_{2}$)