$\forall$$A$, $B$:Type, $f$, $g$:($A$$\rightarrow$($B$ + Top)), $x$:$A$.
\\[0ex]($\uparrow$can{-}apply([$f$?$g$];$x$)) $\Leftarrow\!\Rightarrow$ (($\uparrow$can{-}apply($f$;$x$)) $\vee$ ($\uparrow$can{-}apply($g$;$x$)))