proposition: Lemma orSym (T : System) (f g : Formula): SysPrf T (f \/ g)%fol -> SysPrf T (g \/ f)%fol.
proof: Proof. intros H; eapply orE. - apply H. - apply impI, orI2, Axm; right; constructor. - apply impI, orI1, Axm; right; constructor. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)