(* TO BE A DOUBLE-LINK: *) 

Definition double_link(m:fmap)(x x':dart):Prop:=
  x <> x' /\ expe m x x'.

Lemma double_link_succ :forall (m:fmap)(x x':dart), inv_hmap m -> double_link m x x' -> (succ m zero x \/ succ m zero x').