proposition: Lemma iffI (T : System) (f g : Formula) : SysPrf T (f -> g)%nt -> SysPrf T (g -> f)%nt -> SysPrf T (f <-> g)%nt.
proof: Proof. apply (iffI LNT). Qed.
imports: Coq(Arith Ensembles List) hydras.Ackermann(Languages folProof folProp folLogic3)