$\forall$$T$:Type, $l_{1}$, $l_{2}$:$T$ List, $x$:$T$. l\_disjoint($T$;$l_{1}$;$l_{2}$) $\Rightarrow$ ($x$ $\in$ $l_{1}$) $\Rightarrow$ $\neg$($x$ $\in$ $l_{2}$)