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 Forall_Znth:
forall {A}{a: Inhabitant A} (F: A -> Prop) (al: list A) i,
0 <= i < Zlength al ->
Forall F al ->
F (Znth i al).