$\vdash$  $\forall$$T$:Type, $P_{1}$, $P_{2}$:($T$$\rightarrow\mathbb{B}$), $L$:($T$ List). filter($P_{1}$;filter($P_{2}$;$L$)) $\sim$ filter($P_{2}$;filter($P_{1}$;$L$))