(** All the proofs that follow duplicate [Multiset_of_A] *)
(** Here we should make uniset an abstract datatype, by hiding [Charac], [union], [charac]; all further properties are proved abstractly *)

Lemma uniset_twist2 :
forall x y z t:uniset,
seq (union x (union (union y z) t)) (union (union y (union x z)) t).