proposition: Lemma iffTrans (T : System) (f g h : Formula): SysPrf T (f <-> g)%fol -> SysPrf T (g <-> h)%fol -> SysPrf T (f <-> h)%fol.
proof: Proof. intros H H0; apply iffI. - eapply impTrans. + apply iffE1, H. + apply iffE1, H0. - eapply impTrans. + apply iffE2, H0. + apply iffE2, H. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)