Goal:
  
  A : Type
  a : A
  l : list A
  n : nat
  IHl : ∀ n : nat, firstn n (firstn (S n) l) = firstn n l
  ============================
   (a :: firstn n l = a :: firstn 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"