apply ImpI;is_ass.
eapply AndI.
apply Clause_provable;apply H;constructor;reflexivity.
apply IHll;intro;intro;apply H;constructor 2;assumption.
Qed.
(** Proof that (CNF A) → (NNF A) is provable*)

Lemma prov_or : forall A1 A2 B1 B2 Γ, Provable (A1 → A2) -> Provable (B1 → B2) -> In (A1∨B1) Γ -> Γ ⊢ A2∨B2.
intros; prov_impl_in H;prov_impl_in H0. eapply OrE.