compose{-}fpf($a$; $b$; $f$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$<$mapfilter(($\lambda$$x$.outl($a$($x$))); ($\lambda$$x$.isl($a$($x$))); fpf{-}domain($f$)), compose(($f$.2); $b$)$>$