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_16_2 : forall A B C U X, A<>B -> Col A B X -> Col A B U -> U<>X -> ~ Col A B C -> Per C X U ->Perp A B C X.