$\forall$$T$:Type, $B$:($T$$\rightarrow$(Top + Top)), $L$:($T$ List).
\\[0ex]concat(map($\lambda$$x$.case $B$($x$) of inl($a$) =$>$ [$a$] $\mid$ inr($a$) =$>$ [];$L$))
\\[0ex]$\sim$
\\[0ex]mapfilter($\lambda$$x$.outl($B$($x$));$\lambda$$x$.isl($B$($x$));$L$)