$\forall$$T$:Type, $g$:Top, $L$:$T$ List, $f$:(\{$x$:$T$$\mid$ ($x$ $\in$ $L$) \}$\rightarrow$(Top+Top)).
\\[0ex]concat(map($\lambda$$x$.map($\lambda$$y$.$g$($x$,$y$);Case $f$($x$) of inl($m$) $\Rightarrow$ [$m$] ; inr($x$) $\Rightarrow$ nil);$L$))
\\[0ex]$\sim$
\\[0ex]mapfilter($\lambda$$x$.$g$($x$,outl($f$($x$)));$\lambda$$x$.isl($f$($x$));$L$)