Definition removeUndefs (j l j':meminj):meminj :=
fun b => match j b with
None => match l b with
None => None | Some (b1,delta1) => j' b
end
| Some(b2,delta2) => Some(b2, delta2)
end.

Lemma inc_RU: forall j j' (I: inject_incr j j') l,
inject_incr j (removeUndefs j l j').