$\forall$$T$:Type, $P$:($T$$\rightarrow\mathbb{B}$), $l_{1}$,$l_{2}$:($T$ List).
\\[0ex]sqequal(filter($P$; append($l_{1}$; $l_{2}$)); append(filter($P$; $l_{1}$); filter($P$; $l_{2}$)))