$\forall$$T$:Type, ${\it as}$:($T$ List), $f$,$g$:($T$$\rightarrow\mathbb{B}$).
\\[0ex]($\neg$($\uparrow$isl(priority{-}select($f$; $g$; ${\it as}$)))) $\Leftarrow\!\Rightarrow$ l\_all(${\it as}$; $T$; $a$.(($\neg$($\uparrow$($f$($a$)))) $\wedge$ ($\neg$($\uparrow$($g$($a$))))))