Definition Inter := fun X P Q R S =>
  P<>Q /\ R <> S /\ Col X P Q /\ Col X R S /\ 
  ((exists X, Col X P Q /\ ~ Col X R S) \/ (exists X, ~ Col X P Q /\ Col X R S)).

Lemma l6_25 : forall A B, A<>B -> exists C, ~ Col A B C.