repeat rewrite PTree.fold_spec.
simpl in *. repeat rewrite <- fold_left_rev_right.
induction (rev (PTree.elements t2)).
+ reflexivity.
+ simpl. rewrite IHl. auto.
Qed.

Lemma LocalD_to_localD : forall P R t l,
PROPx (P) (LOCALx (LocalD t l []) (SEPx (R))) =
PROPx (P) (LOCALx (localD t l) (SEPx (R))).