Lemma RInt_comp (f : R -> V) (g dg : R -> R) (a b : R) : (forall x, Rmin a b <= x <= Rmax a b -> continuous f (g x)) -> (forall x, Rmin a b <= x <= Rmax a b -> is_derive g x (dg x) /\ continuous dg x) -> RInt (fun y => scal (dg y) (f (g y))) a b = RInt f (g a) (g b).