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:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"