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 fold_right_conj :
forall a b rho,
(fold_right (fun (x0 x1 : environ -> Prop) (x2 : environ) => x0 x2 /\ x1 x2)
(`True) (a ++ b) rho) <-> ((fold_right (fun (x0 x1 : environ -> Prop) (x2 : environ) => x0 x2 /\ x1 x2)
(`True) a rho /\ fold_right (fun (x0 x1 : environ -> Prop) (x2 : environ) => x0 x2 /\ x1 x2)
(`True) b rho)).