Goal:
  
  A : Type
  n : nat
  acc : list A
  IHn : ∀ acc l : list A,
          split_at_aux n acc l = (List.rev acc ++ firstn n l, skipn n l)
  ============================
   ((List.rev acc, []) = (List.rev acc ++ [], []))


DEBUG:Going to execute:
rewrite app_nil_r