Fixpoint pow2 (n : nat) : nat :=
  match n with
    | O => 1%nat
    | S n => (2 * pow2 n)%nat
  end.

Lemma sum_f_R0_skip (u : nat -> R) (n : nat) : sum_f_R0 (fun k => u (n - k)%nat) n = sum_f_R0 u n.