Hint Rewrite
(@Znth_map Z _) (@Znth_map nat _) (@Znth_map positive _)
using (auto; rewrite ?Zlength_map in *; omega) : sublist.
(* Add these in a later file where things are in scope ... Hint Rewrite (Znth_map Int.zero) (Znth_map Vundef) using (auto; rewrite ?Zlength_map in *; omega) : sublist. *)

Lemma Znth_succ: forall {A}{a: Inhabitant A} i lo (v: list A), Z.succ lo <= i -> Znth (i - lo) v = Znth (i - (Z.succ lo)) (skipn 1 v).