Definition agree64 (a: Ptrofs.int) (b: Int64.int) : Prop :=
  Ptrofs.unsigned a = Int64.unsigned b.

Lemma agree64_sub: forall a1 b1 a2 b2, agree64 a1 b1 -> agree64 a2 b2 -> agree64 (Ptrofs.sub a1 a2) (Int64.sub b1 b2).