(*
 Lemma agree_except_sym:
  forall x s s', agree_except x s s' -> agree_except x s' s.
 Proof.
 unfold agree_except; intuition. symmetry. apply H0; auto.
 Qed.
*)

Axiom env_reset2: forall s x z, env_set x (env_get s x) (env_set x z s) = s.

Lemma or_FF: forall {A} (P: pred A), (orp P FF) = P.