Definition primitive (f:R -> R) (a b:R) (h:a <= b)
  (pr:forall x:R, a <= x -> x <= b -> Riemann_integrable f a x)
  (x:R) : R :=
  match Rle_dec a x with
    | left r =>
      match Rle_dec x b with
        | left r0 => RiemannInt (pr x r r0)
        | right _ => f b * (x - b) + RiemannInt (pr b h (Rle_refl b))
      end
    | right _ => f a * (x - a)
  end.

Lemma RiemannInt_P28 : forall (f:R -> R) (a b x:R) (h:a <= b) (C0:forall x:R, a <= x <= b -> continuity_pt f x), a <= x <= b -> derivable_pt_lim (primitive h (FTC_P1 h C0)) x (f x).