(** uniset union *)
Definition union (m1 m2:uniset) :=
  Charac (fun a:A => orb (charac m1 a) (charac m2 a)).

Lemma treesort_twist2 : forall x y z t u:uniset, seq u (union y z) -> seq (union x (union u t)) (union (union y (union x z)) t).