$\forall$$T$:Type, ${\it as}$:($T$ List), ${\it bs}$:(Top List), $P$:(\{$x$:$T$$\mid$ ($x$ $\in$ ${\it as}$)\} $\rightarrow\mathbb{B}$), $f$:Top.
\\[0ex]mapfilter($f$;$P$;${\it as}$ @ ${\it bs}$) $\sim$ (mapfilter($f$;$P$;${\it as}$) @ mapfilter($f$;$P$;${\it bs}$))