(** The predicate out P A B informally means that P is on the line A B outside the segment A B. *) 
  
Definition out := fun P A B => A<>P /\ B<>P /\ (Bet P A B \/ Bet P B A).

Lemma l6_11_unicity : forall A B C R X Y, R<>A -> B<>C -> out A X R -> Cong A X B C -> out A Y R -> Cong A Y B C -> X=Y.