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 frgnSrc: forall mu (WD: SM_wd mu) b1, frgnBlocksSrc mu b1 = true ->
exists b2 z, foreign_of mu b1 = Some(b2,z) /\
frgnBlocksTgt mu b2 = true.