proposition: Lemma sysWeaken (T : System) (f g : Formula): SysPrf T f -> SysPrf (Ensembles.Add T g) f.
proof: Proof. intros H; apply sysExtend with T. - intros f0 Hf0; now left. - assumption. Qed.
imports: Coq(Ensembles List) hydras.Ackermann(ListExt folProof folProp Deduction)