Transparent Z.of_nat.

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)).