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


DEBUG:Going to execute:
t; easy