Definition DistLn := fun A B C D => 
(exists X, Col X A B /\ ~Col X C D) \/ (exists X, ~ Col X A B /\ Col X C D).

Lemma l8_14_2_1b_bis : forall A B C D X, Perp A B C D -> Col X A B -> Col X C D -> Perp_in X A B C D.