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(* modify it under the terms of the GNU Lesser General Public License *)
(* as published by the Free Software Foundation; either version 2.1   *)
(* of the License, or (at your option) any later version.             *)
(*                                                                    *)
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(* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the      *)
(* GNU General Public License for more details.                       *)
(*                                                                    *)
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(* Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA *)
(* 02110-1301 USA                                                     *)
(* conservation de l'invariant de compatibilité par rapport à une signature pour l'union *)
(* conservation de l'invariant ref_ok pour l'union *)

Lemma union_pl_ref_ok_invar :
forall (p p' : prec_list) (d : preDTA),
prec_list_ref_ok p d ->
prec_list_ref_ok p' d -> prec_list_ref_ok (union_pl p p') d.