(* Lemma t2_8 : forall A B C D E : Tpoint, Bet A B C -> Bet D B E -> Cong A B D B -> Cong B C B E -> Cong A E C D. *)

Lemma col3 : forall X Y A B C,
X <> Y ->
Col X Y A -> Col X Y B -> Col X Y C ->
Col A B C.