Inductive pointer_val : Type :=
  | ValidPointer: block -> Ptrofs.int -> pointer_val
  | NullPointer.Inductive pointer_val : Type :=
  | ValidPointer: block -> Ptrofs.int -> pointer_val
  | NullPointer.Inductive pointer_val : Type :=
  | ValidPointer: block -> Ptrofs.int -> pointer_val
  | NullPointer.

(* Specifications: *)
Lemma PV_eq_dec: forall x y: pointer_val, {x = y} + {x <> y}.
Lemma zero_in_range : (-1 < 0 < Int.modulus)%Z.