fact: Axiom def_para_plan_dr2 : forall A B C F G : PO, ~ alignes A B C -> F <> G -> para_plan_dr (plan A B C) (droite F G) -> exists D : PO, (exists E : PO, (coplanaires A B C D /\ coplanaires A B C E) /\ D <> E /\ paralleles (droite D E) (droite F G)).