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 locBlocksSrc_externNone: forall mu (WD:SM_wd mu) b
(MB: locBlocksSrc mu b = true), extern_of mu b = None.