(** ** Lemmas about erased values*)

  Definition isDefined v :=
    match v with
    | Vundef => false
    | _ => true
    end.

Lemma val_erasure_xorl: forall v1 v2 v1' v2', val_erasure v1 v1' -> val_erasure v2 v2' -> val_erasure (Val.xorl v1 v2) (Val.xorl v1' v2').