Goal:
  
  A : Type
  B : Type
  n : nat
  f : nat → A → B
  k : nat
  a : A
  l : list A
  IHn : ∀ (f : nat → A → B) (k : nat) (l : list A),
          skipn n (mapi_rec f l k) = mapi_rec f (skipn n l) (n + k)
  ============================
   (mapi_rec f (skipn n l) (S (n + k)) = mapi_rec f (skipn n l) (S (n + k)))


DEBUG:Going to execute:
let rec use_hyps :=
            match goal with
            | H:_ ∧ _ |- _ => exact H || (destruct_hyp H; use_hyps)
            | H:_ * _ |- _ => exact H || (destruct_hyp H; use_hyps)
            | H:_ |- _ => solve [ inversion H ]
            | _ => idtac
            end
           in
           let do_atom := solve
           [ trivial
           with eq_true
           | reflexivity
           | symmetry; trivial
           | contradiction
           | congruence ]
           in
           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"