diff --git a/Foundation/FirstOrder/Incompleteness/Essentially.lean b/Foundation/FirstOrder/Incompleteness/Essentially.lean
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+++ b/Foundation/FirstOrder/Incompleteness/Essentially.lean
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+import Foundation.FirstOrder.Incompleteness.First
+
+namespace LO.FirstOrder
+
+
+namespace ArithmeticAxiom
+
+variable {T : ArithmeticAxiom} {i : ℕ}
+
+open Classical
+open Encodable
+
+protected def computableExpansion.next (T : ArithmeticAxiom) (σ : ArithmeticSentence) : ArithmeticAxiom := if Entailment.Consistent (insert σ T) then (insert σ T) else (insert (∼σ) T)
+
+protected def computableExpansion.indexed (T : ArithmeticAxiom) : ℕ → ArithmeticAxiom
+  | 0 => T
+  | i + 1 =>
+    match (decode i) with
+    | some σ => computableExpansion.next (computableExpansion.indexed T i) σ
+    | none => computableExpansion.indexed T i
+
+local notation:max T"[" i "]" => computableExpansion.indexed T i
+
+namespace computableExpansion
+
+lemma indexed_mem_either : σ ∈ T[(encode σ) + 1] ∨ ∼σ ∈ T[(encode σ) + 1] := by
+  simp only [encodek, computableExpansion.indexed, computableExpansion.next];
+  split <;> tauto;
+
+lemma indexed_provable_either : T[(encode σ) + 1] ⊢! σ ∨ T[(encode σ) + 1] ⊢! ∼σ := by
+  rcases indexed_mem_either (T := T) (σ := σ) with (h | h);
+  . left;
+    apply Entailment.Axiomatized.provable_axm;
+    simpa;
+  . right;
+    apply Entailment.Axiomatized.provable_axm;
+    simpa;
+
+lemma indexed_consistent_next : Entailment.Consistent T[i] → Entailment.Consistent T[i + 1] := by
+  simp only [computableExpansion.indexed, computableExpansion.next];
+  intro h;
+  split;
+  . split;
+    . simpa;
+    . sorry;
+  . assumption;
+
+lemma indexed_consistent [Entailment.Consistent T] : Entailment.Consistent T[i] := by
+  induction i with
+  | zero => simpa [computableExpansion.indexed];
+  | succ i hi => apply indexed_consistent_next hi;
+
+lemma indexed_weakerThan_next : T[i] ⪯ T[i + 1] := by
+  simp only [computableExpansion.indexed, computableExpansion.next];
+  split;
+  . split <;>
+    . apply Entailment.Axiomatized.weakerThanOfSubset;
+      simp;
+  . tauto;
+
+lemma indexed_weakerThan_original : T ⪯ T[i] := by
+  induction i with
+  | zero => simp [computableExpansion.indexed];
+  | succ i hi =>
+    calc
+      T ⪯ T[i]     := by simpa;
+      _ ⪯ T[i + 1] := indexed_weakerThan_next;
+
+end computableExpansion
+
+def computableExpansion (T : ArithmeticAxiom) : ArithmeticAxiom := ⋃ i, T[i]
+
+local notation:max T"∞" => computableExpansion T
+
+namespace computableExpansion
+
+instance weakerThan_indexed : T[i] ⪯ T∞ := by
+  apply Entailment.Axiomatized.weakerThanOfSubset;
+  intro σ hσ;
+  simp only [computableExpansion, Set.mem_iUnion];
+  use i;
+
+instance weakerThan_original : T ⪯ T∞ := by
+  nth_rw 1 [(show T = T[0] by rfl)];
+  apply weakerThan_indexed;
+
+lemma consistent_of_consistent : Entailment.Consistent T → Entailment.Consistent T∞ := by
+  simp [Entailment.consistent_iff_exists_unprovable]
+  intro σ hσ;
+  use σ;
+  sorry;
+
+instance [Entailment.Consistent T] : Entailment.Consistent T∞ := by
+  apply consistent_of_consistent;
+  infer_instance;
+
+lemma complete : Entailment.Complete T∞ := by
+  intro σ;
+  rcases indexed_provable_either with (h | h);
+  . left; apply weakerThan_indexed.pbl h;
+  . right; apply weakerThan_indexed.pbl h;
+
+end computableExpansion
+
+end ArithmeticAxiom
+
+
+
+lemma Arithmetic.iff_essentially_incomplete_essentially_undecidable (T : ArithmeticAxiom) :
+  -- TODO: [Theory.Δ₁ U.toTheory] should be rewrited to r.e.
+  (∀ U : ArithmeticAxiom, T ⪯ U → [Entailment.Consistent U] → [Theory.Δ₁ U.toTheory] → ¬Entailment.Complete (U : Axiom ℒₒᵣ))
+  ↔
+  (∀ U : ArithmeticAxiom, T ⪯ U → [Entailment.Consistent U] → ¬ComputablePred (fun n : ℕ ↦ ∃ σ : ArithmeticSentence, n = ⌜σ⌝ ∧ U ⊢! σ))
+  := by
+  constructor;
+  . intro h S _ _;
+    sorry;
+  . contrapose!;
+    rintro ⟨U, _, _, _, hU⟩;
+    use U.computableExpansion;
+    refine ⟨?_, ?_, ?_⟩;
+    . calc
+        T ⪯ U := by assumption;
+        _ ⪯ U.computableExpansion := inferInstance;
+    . infer_instance;
+    . sorry;
+
+end LO.FirstOrder