Goal:
  
  A : Type
  a : A
  l : list A
  l' : list A
  IHl : ∀ v : nat, #|l| ≤ v → nth_error (l ++ l') v = nth_error l' (v - #|l|)
  H : S #|l| ≤ 0
  ============================
   (nth_error (a :: l ++ l') 0 = nth_error l' (0 - S #|l|))


DEBUG:Going to execute:
PTRDEBUGTACintros; Zify.zify_pre_hook; zify_elim_let; zify_op;
            Zify.zify_iter_specs; Zify.zify_to_euclidean_division_equations;
            Zify.zify_post_hook; zify_saturate