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

Lemma Forall2_Znth : forall {A B}{d1: Inhabitant A}{d2: Inhabitant B} (P : A -> B -> Prop) l1 l2 (Hall : Forall2 P l1 l2) i (Hi : 0 <= i < Zlength l1), P (Znth i l1) (Znth i l2).