(** A generalization of the Cong predicate for three points. *)

Definition Cong_3 := fun A1 A2 A3 B1 B2 B3 =>
  Cong A1 A2 B1 B2 /\ Cong A1 A3 B1 B3 /\ Cong A2 A3 B2 B3.

Lemma col_permutation_4 : forall A B C, Col A B C -> Col B A C.