$\forall$$T$:Type, ${\it eq}$:EqDecider($T$), $a$, $b$, $c$:($T$ List).
\\[0ex](l\_disjoint($T$;$b$;$a$) $\vee$ l\_disjoint($T$;$c$;$a$)) $\Rightarrow$ l\_disjoint($T$;l\_intersection(${\it eq}$;$b$;$c$);$a$)