proposition: Lemma nAnd (T : System) (f g : Formula): SysPrf T (~ f \/ ~ g)%fol -> SysPrf T (~ (f /\ g))%fol.
proof: Proof. intros H; unfold andH; apply nnI, H. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)