(* Notion of bijection: *)

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

Lemma B_1_not_exd : forall (m:fmap)(k:dim)(x:dart), inv_hmap m -> ~exd m x -> B_1 m k x = m.