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