(* Another characterization of R* *)
(* Smallest reflexive P containing R o R* *)

Definition Rstar' (x y : A) :=
  forall P : A -> A -> Prop,
  P x x -> (forall u : A, R x u -> Rstar u y -> P x y) -> P x y.

Theorem Rstar'_R : forall x y z : A, R x z -> Rstar z y -> Rstar' x y.