proposition: Lemma absurd1 (T : System) (f : Formula) : SysPrf T (f -> ~f)%fol -> SysPrf T (~f)%fol.
proof: Proof. intros H; apply orE with (~f)%fol f. - apply noMiddle. - apply impI; apply Axm; right; constructor. - assumption. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)