(** ** Elements *)

(** The converse of [elements_spec2], used in MSetRBT *)
(* TODO: TO MIGRATE ELSEWHERE... *)
Lemma sorted_app_inv l1 l2 :
sort X.lt (l1++l2) ->
sort X.lt l1 /\ sort X.lt l2 /\
forall x1 x2, InA X.eq x1 l1 -> InA X.eq x2 l2 -> X.lt x1 x2.