fact: Axiom def_secants : forall A B C D E F I J : PO, ~ alignes A B C -> ~ alignes D E F -> I <> J -> incluse (droite I J) (plan A B C) -> incluse (droite I J) (plan D E F) -> ~ para_plan_plan (plan A B C) (plan D E F) -> secants (plan A B C) (plan D E F).