proposition: Lemma DN A phi : A ⊢C (¬¬phi) -> A ⊢C phi.
proof: Proof. intros H. eapply IE with ((phi → falsity) → phi); try apply Pc. apply II, Exp. eapply IE. apply (Weak H); auto. now apply Ctx. Qed.
imports: Undecidability.PCP(PCP Util.PCP_facts) Undecidability.FOL(Deduction.FragmentNDFacts Semantics.Tarski.FragmentFacts Semantics.Tarski.FragmentSoundness Syntax.Core Syntax.Facts) Undecidability.FOL(FOL) Undecidability.Synthetic(Definitions DecidabilityFacts EnumerabilityFacts ReducibilityFacts MoreReducibilityFacts) Undecidability.FOL.Reductions.PCPb_to_FOL Undecidability.PCP.Reductions.PCPb_iff_dPCPb List Undecidability(Shared.ListAutomation)