proposition: Lemma existI (T : System) (f : Formula) (v : nat) (t : Term): SysPrf T (substF f v t) -> SysPrf T (exH v, f)%fol.
proof: Proof. intros H; unfold existH, fol.existH in |- *; apply impE with (~ ~ (substF f v t))%fol. - apply cp2, impI; rewrite <- (subFormulaNot L). apply forallE, Axm; right; constructor. - now apply nnI. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)