$\forall$$A$, $B$:Type, ${\it xs}$, ${\it ys}$:$A$ List. strong{-}subtype($A$;$B$) $\Rightarrow$ \{${\it xs}$ $\leq$ ${\it ys}$ $\Rightarrow$ ${\it xs}$ $\leq$ ${\it ys}$\}