Remark compress_Acc:
  forall x,
  Acc (order uf.(m)) x -> Acc (order (M.set a b uf.(m))) x.

(* Specifications: *)
Lemma compress_wf: well_founded (order (M.set a b uf.(m))).
Lemma compress_wf: well_founded (order (M.set a b uf.(m))).