proposition: Lemma nExist (T : System) (f : Formula) (v : nat): SysPrf T (allH v, ~f)%fol -> SysPrf T (~ (exH v, f))%fol.
proof: Proof. intros H; now apply nnI. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)