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_left:
forall j k b b' delta (J: j b = Some (b',delta))
(D:disjoint j k), k b = None.