Definition val_inject_opt (j: meminj) (v1 v2: option val) :=
  match v1, v2 with Some v1', Some v2' => val_inject j v1' v2'
  | None, None => True
  | _, _ => False
  end.

Lemma extends_permorder: forall m1 m2 (Ext: Mem.extends m1 m2) (b:block) ofs k, Mem.perm_order'' (PMap.get b (Mem.mem_access m2) ofs k) (PMap.get b (Mem.mem_access m1) ofs k).