proposition: Lemma iffSym (T : System) (f g : Formula) : SysPrf T (f <-> g)%fol -> SysPrf T (g <-> f)%fol.
proof: Proof. intro H; now apply andSym. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)