(* Notion of bijection: *)
Definition bij_dart(Df:dart->Prop)(f:dart->dart):Prop:=              inj_dart Df f /\ surj_dart Df f.
(* (cA m k) is bijective on (exd m): *)

Theorem bij_cA : forall (m:fmap)(k:dim), inv_hmap m ->
bij_dart (exd m) (cA m k).