(* Notion of bijection: *)

Definition bij_dart(Df:dart->Prop)(f:dart->dart):Prop:=              inj_dart Df f /\ surj_dart Df f.

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.