(** Five segments configuration. *)

Definition FSC := fun A B C D A' B' C' D' =>
  Col A B C /\ Cong_3 A B C A' B' C' /\ Cong A D A' D' /\ Cong B D B' D'.

Lemma l4_19 : forall A B C C', Bet A C B -> Cong A C A C' -> Cong B C B C' -> C=C'.