Notation Union := (union A R1 R2).Remark strip_commut :
    commut A R1 R2 ->
    forall x y:A,
      clos_trans A R1 y x ->
      forall z:A, R2 z y ->  exists2 y' : A, R2 y' x & clos_trans A R1 z y'.

Lemma Acc_union : commut A R1 R2 -> (forall x:A, Acc R2 x -> Acc R1 x) -> forall a:A, Acc R2 a -> Acc Union a.