(* The following hint has been disabled since it isn't necessary (cf. cong_inner_transitivity + cong_symmetry) and it may interact badly with cong_inner_transitivity to create proofs with remaining evars *)
(*Hint Resolve cong_transitivity : Tarski.*)

Lemma cong_left_commutativity : forall A B C D,
Cong A B C D -> Cong B A C D.