proposition: Lemma iffE2 (T : System) (f g : Formula): SysPrf T (f <-> g)%fol -> SysPrf T (g -> f)%fol.
proof: Proof. intros H; eapply andE2, H. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)