Hint Rewrite @app_nil_l @app_nil_r : sublist.

Lemma Znth_cons:
forall {A}(a: Inhabitant A)  i (al: list A) bl,
0 <= i < Zlength al  ->
Znth i al :: bl = firstn (Z.to_nat 1) (skipn (Z.to_nat i) al) ++ bl.