Definition mem_forward (m1 m2:mem) :=
  forall b, Mem.valid_block m1 b ->
    (Mem.valid_block m2 b
     /\ (forall ofs p, Mem.perm m2 b ofs Max p -> Mem.perm m1 b ofs Max p)
     (*/\ readonly m1 b m2*)).

Lemma compose_meminjD_None: forall j jj b, (compose_meminj j jj) b = None -> j b = None \/ (exists b', exists ofs, j b = Some(b',ofs) /\ jj b' = None).