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