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 zero_in_range : (-1 < 0 < Int.modulus)%Z.