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 joinI: forall f g b p
(Hp: f b = Some p \/ (f b = None /\ g b = Some p)),
join f g b = Some p.