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