proposition: Lemma existSimp (T : System) (f : Formula) (v : nat) : SysPrf T f -> SysPrf T (exH v, f)%fol.
proof: Proof. intros H; eapply existI; now rewrite subFormulaId. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)