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