Definition extension_C1 (f df : R -> V) (a b : Rbar) (x : R) : V :=
  match Rbar_le_dec a x with
    | left _ => match Rbar_le_dec x b with
        | left _ => f x
        | right _ => plus (f (real b)) (scal (x - real b) (df (real b)))
      end
    | right _ => plus (f (real a)) (scal (x - real a) (df (real a)))
  end.

Lemma extension_C1_is_derive (f df : R -> V) (a b : Rbar) : Rbar_le a b -> (forall x : R, Rbar_le a x -> Rbar_le x b -> is_derive f x (df x)) -> forall x : R, is_derive (extension_C1 f df a b) x (extension_C0 df a b x).