(* Lemma RInt_comp_opp (f : R -> V) (a b : R) : ex_RInt f (-a) (-b) -> RInt (fun y => f (- y)) a b = opp (RInt f (-a) (-b)). *)

Lemma RInt_Chasles :
forall f a b c,
ex_RInt f a b -> ex_RInt f b c ->
plus (RInt f a b) (RInt f b c) = RInt f a c.