Proof with Tarski.
intros.
unfold out.
intuition.
subst A.
assert (Bet P P B)...
subst B.
assert (Bet A P P)...
unfold Col in H0;intuition.
Qed.
(** out reflexivity. *)

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.