(* invariant sur ref_ok pour le kill_empty *)

Lemma preDTA_kill_ref_ok_distinct_invar :
forall (d : preDTA) (sigma : signature),
preDTA_ref_ok_distinct d d ->
predta_correct_wrt_sign d sigma ->
preDTA_ref_ok_distinct (preDTA_kill (dta_non_empty_states d) d)
(preDTA_kill (dta_non_empty_states d) d).