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'.

Theorem wf_union : commut A R1 R2 -> well_founded R1 -> well_founded R2 -> well_founded Union.