$\forall$$A$,$B$:Type, ${\it xs}$,${\it ys}$:($A$ List).
\\[0ex]strong{-}subtype($A$; $B$) $\Rightarrow$ guard((iseg($B$; ${\it xs}$; ${\it ys}$) $\Rightarrow$ iseg($A$; ${\it xs}$; ${\it ys}$)))