(* DEFINITION OF eqco WHICH ALLOWS TO PUT AHEAD ANY 0-LINK: *)
Definition eqco(m:fmap)(k:dim)(x z t:dart):=
eqc (L (B m k x) k x (A m k x)) z t.

Lemma eqco_eqc:forall(m:fmap)(k:dim)(x z t:dart),
inv_hmap m -> succ m k x ->
(eqco m k x z t <-> eqc m z t).