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.

Lemma PV_eq_dec: forall x y: pointer_val, {x = y} + {x <> y}.