fact: Axiom def_secants2 : forall A B C D E F : PO, ~ alignes A B C -> ~ alignes D E F -> secants (plan A B C) (plan D E F) -> ~ para_plan_plan (plan A B C) (plan D E F) /\ (exists I : PO, (exists J : PO, I <> J /\ incluse (droite I J) (plan A B C) /\ incluse (droite I J) (plan D E F))).