(* TEST: OK ... Lemma A_1_B_1_bis_bis : forall (m:fmap)(k:dim)(x y:dart), y <> x -> A_1 (B_1 m k x) k y = A_1 m k y. *)
(* Action_ter of B_1 on A: *)

Lemma A_1_B_1_ter : forall (m:fmap)(k j:dim)(x y:dart),
k<>j -> A_1 (B_1 m k x) j y = A_1 m j y.