Definition nat_to_perm (i:nat) :=
  (match i with
  | 0 => Some None
  | 1 => Some (Some Nonempty)
  | 2 => Some (Some Readable)
  | 3 => Some (Some Writable)
  | 4 => Some (Some Freeable)
  | _ => None
  end)%nat.Definition perm_to_nat (p: option (option permission)) :=
  match p with
  | Some (None) => 0
  | Some (Some Nonempty) => 1
  | Some (Some Readable) => 2
  | Some (Some Writable) => 3
  | Some (Some Freeable) => 4
  | None => 5
  end.Definition nat_to_perm_simpl (i:nat) :=
  (match i with
  | 0 => None
  | 1 => Some Nonempty
  | 2 => Some Readable
  | 3 => Some Writable
  | 4 => Some Freeable
  | _ => None
  end)%nat.Definition perm_to_nat_simpl (p: option permission) :=
  match p with
  | None => 0
  | Some Nonempty => 1
  | Some Readable => 2
  | Some Writable => 3
  | Some Freeable => 4
  end.

Lemma perm_to_nat_bound: forall p, perm_to_nat p < 6.