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_0_cons {A}{a: Inhabitant A} l (v:A): Znth 0 (v::l) = v.