proposition: Lemma existI (T : System) (f : Formula) (v : nat) (t : Term): SysPrf T (substF f v t) -> SysPrf T (exH v, f)%nt.
proof: Proof. apply (existI LNT). Qed.
imports: Coq(Arith Ensembles List) hydras.Ackermann(Languages folProof folProp folLogic3)