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)).

Theorem l6_21 : forall A B C D P Q, ~ Col A B C -> C<>D -> Col A B P -> Col A B Q -> Col C D P -> Col C D Q -> P=Q.