Hint Rewrite @Zlength_flip_ends using (autorewrite with sublist; omega) : sublist.

Lemma flip_fact_2:
forall {A}{d: Inhabitant A} (al: list A) size j,
Zlength al = size ->
j < size - j - 1 ->
0 <= j ->
Znth (size - j - 1) al =
Znth (size - j - 1) (flip_ends j (size - j) al).