$\forall$${\it as}$:($\mathbb{Q}$ List). $\exists$$a$:$\mathbb{Q}$. ((($\neg$($\uparrow$null(${\it as}$))) $\Rightarrow$ ($a$ $\in$ ${\it as}$)) \& ($\forall$${\it a'}$$\in$${\it as}$. ${\it a'}$ $\leq$ $a$))