$\forall$$T$:Type, $P$:(($T$ List)$\rightarrow$prop\{i:l\}).
\\[0ex]($\forall$$L$:($T$ List). decidable($P$($L$)))
\\[0ex]$\Rightarrow$ ($\forall$$L$:($T$ List). decidable(($\exists$${\it L'}$:$T$ List. (iseg($T$; ${\it L'}$; $L$) $\wedge$ $P$(${\it L'}$)))))