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. intros H H0; apply andI; auto. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)