(* FINALLY, THE RESULT: OK!! *)

(* Specifications: *)
Theorem nf_L0L0:forall (m:fmap)(x y x' y':dart),
let m1:= L (L m zero x y) zero x' y' in
let m2:= L (L m zero x' y') zero x y in
inv_hmap m1 -> nf m1 = nf m2.