Goal:
  
  A : Type
  x : nat
  y : nat
  IHx : ∀ (y : nat) (args : list A),
          firstn (x + y) args = firstn x args ++ firstn y (skipn x args)
  ============================
   ([] = [])


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"