$\forall$$T$:Type, $a$, $b$, $c$:($T$ List).
\\[0ex]l\_disjoint($T$;$a$;$b$ @ $c$) $\Leftarrow\!\Rightarrow$ (l\_disjoint($T$;$a$;$b$) \& l\_disjoint($T$;$a$;$c$))