proposition: Lemma iffE1 (T : System) (f g : Formula): SysPrf T (f <-> g)%fol -> SysPrf T (f -> g)%fol.
proof: Proof. intros H; eapply andE1. eexact H. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)