$\forall$$T$:Type, $L$:($T$ List), $a$:$T$, ${\it as}$:($T$ List). [$a$ / ${\it as}$] $\subseteq$ $L$ $\Leftarrow\!\Rightarrow$ (($a$ $\in$ $L$) \& ${\it as}$ $\subseteq$ $L$)