l\_disjoint($T$; $l_{1}$; $l_{2}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$x$:$T$. $\neg$(($x$ $\in$ $l_{1}$ $\in$ $T$) $\wedge$ ($x$ $\in$ $l_{2}$ $\in$ $T$))