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_18_unicity : forall A B C X Y, ~ Col A B C -> Col A B X -> Perp A B C X -> Col A B Y -> Perp A B C Y -> X=Y.