proposition: Lemma iffI (T : System) (f g : Formula): SysPrf T (f -> g)%fol -> SysPrf T (g -> f)%fol -> SysPrf T (f <-> g)%fol.
proof: Proof. apply (iffI LNN). Qed.
imports: Coq(Arith Ensembles List) LibHyps(LibHyps) hydras(MoreLibHyps NewNotations)