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 fwd_maxperm: forall m1 m2 (FWD: mem_forward m1 m2) b (V:Mem.valid_block m1 b) ofs p, Mem.perm m2 b ofs Max p -> Mem.perm m1 b ofs Max p.