Corollary NoDup_app_swap : forall {A} (l1 l2 : list A), NoDup (l1 ++ l2) <-> NoDup (l2 ++ l1).

Lemma NoDup_concat_less : forall {A} l i (l1 l2 : list A) (Hl : NoDup (concat l)) (Hi : 0 <= i < Zlength l) (Hless : Znth i l = l1 ++ l2), NoDup (concat (upd_Znth i l l1)).