proposition: Lemma andSym (T : System) (f g : Formula): SysPrf T (f /\ g)%fol -> SysPrf T (g /\ f)%fol.
proof: Proof. intros H; apply andI. - eapply andE2; apply H. - eapply andE1; apply H. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)