Definition join (j k:meminj):meminj := fun b =>
  match j b with
     Some (b1, delta) => Some (b1,delta)
   | None => k b
  end.Definition disjoint (j k:meminj):Prop :=
    forall b, j b = None \/ k b = None.

Lemma disjointD_right: forall j k b b' delta (K: k b = Some (b',delta)) (D:disjoint j k), j b = None.