diff --git a/ConNF/BaseType/Params.lean b/ConNF/BaseType/Params.lean
index 964c22982a..6253e8f5d8 100644
--- a/ConNF/BaseType/Params.lean
+++ b/ConNF/BaseType/Params.lean
@@ -331,6 +331,10 @@ theorem κ_not_lt_zero (i : κ) : ¬i < 0 := by
   · exact h.not_lt
   · exact h.not_lt
 
+theorem κ_pos (i : κ) : 0 ≤ i := by
+  rw [← not_lt]
+  exact κ_not_lt_zero i
+
 @[simp]
 theorem κ_add_eq_zero_iff (i j : κ) : i + j = 0 ↔ i = 0 ∧ j = 0 :=
   by rw [← Ordinal.typein_inj (α := κ) (· < ·), ← Ordinal.typein_inj (α := κ) (· < ·),
diff --git a/ConNF/Structural/Enumeration.lean b/ConNF/Structural/Enumeration.lean
index 3e47e04860..acac798736 100644
--- a/ConNF/Structural/Enumeration.lean
+++ b/ConNF/Structural/Enumeration.lean
@@ -2,7 +2,7 @@ import Mathlib.Data.Set.Pointwise.SMul
 import ConNF.BaseType.Small
 import ConNF.Structural.Index
 
-open Cardinal
+open Cardinal Ordinal
 
 open scoped Cardinal Pointwise
 
@@ -37,6 +37,10 @@ theorem Enumeration.mem_iff (c : α) (E : Enumeration α) :
     c ∈ E ↔ ∃ i, ∃ (h : i < E.max), c = E.f i h :=
   Iff.rfl
 
+theorem Enumeration.f_mem (E : Enumeration α) (i : κ) (hi : i < E.max) :
+    E.f i hi ∈ E :=
+  ⟨i, hi, rfl⟩
+
 theorem Enumeration.carrier_small (E : Enumeration α) : Small E.carrier := by
   refine lt_of_le_of_lt (b := #(Set.Iio E.max)) ?_ (card_typein_lt (· < ·) E.max Params.κ_ord.symm)
   refine mk_le_of_surjective (f := fun x => ⟨E.f x x.prop, x, x.prop, rfl⟩) ?_
@@ -97,11 +101,11 @@ theorem mk_fun_le {α β : Type u} :
   let _ := linearOrderOfSTO r
   exact ⟨⟨funMap α β, funMap_injective⟩⟩
 
-theorem pow_le_of_isEtrongLimit' {α β : Type u} [Infinite α] [Infinite β]
+theorem pow_le_of_isStrongLimit' {α β : Type u} [Infinite α] [Infinite β]
     (h₁ : IsStrongLimit #β) (h₂ : #α < (#β).ord.cof) : #β ^ #α ≤ #β := by
   refine le_trans mk_fun_le ?_
-  simp only [mk_prod, lift_id, mk_pi, mk_fintype, Fintype.card_prop, Nat.cast_ofNat, prod_const,
-    lift_id', lift_two]
+  simp only [mk_prod, Cardinal.lift_id, mk_pi, mk_fintype, Fintype.card_prop, Nat.cast_ofNat,
+    prod_const, Cardinal.lift_id', lift_two]
   have h₃ : #{ E : Set β // #E ≤ #α } ≤ #β
   · rw [← mk_subset_mk_lt_cof h₁.2]
     refine ⟨⟨fun E => ⟨E, E.prop.trans_lt h₂⟩, ?_⟩⟩
@@ -111,12 +115,12 @@ theorem pow_le_of_isEtrongLimit' {α β : Type u} [Infinite α] [Infinite β]
   have h₄ : (2 ^ #α) ^ #α ≤ #β
   · rw [← power_mul, mul_eq_self (Cardinal.infinite_iff.mp inferInstance)]
     refine (h₁.2 _ ?_).le
-    exact h₂.trans_le (Ordinal.cof_ord_le #β)
+    exact h₂.trans_le (cof_ord_le #β)
   refine le_trans (mul_le_max _ _) ?_
   simp only [ge_iff_le, le_max_iff, max_le_iff, le_aleph0_iff_subtype_countable, h₃, h₄, and_self,
     aleph0_le_mk]
 
-theorem pow_le_of_isEtrongLimit {κ μ : Cardinal.{u}} (h₁ : IsStrongLimit μ) (h₂ : κ < μ.ord.cof) :
+theorem pow_le_of_isStrongLimit {κ μ : Cardinal.{u}} (h₁ : IsStrongLimit μ) (h₂ : κ < μ.ord.cof) :
     μ ^ κ ≤ μ := by
   by_cases h : κ < ℵ₀
   · exact pow_le h₁.isLimit.aleph0_le h
@@ -125,20 +129,21 @@ theorem pow_le_of_isEtrongLimit {κ μ : Cardinal.{u}} (h₁ : IsStrongLimit μ)
     intro α β h₁ h₂ h
     have := Cardinal.infinite_iff.mpr (le_of_not_lt h)
     have := Cardinal.infinite_iff.mpr h₁.isLimit.aleph0_le
-    exact pow_le_of_isEtrongLimit' h₁ h₂
+    exact pow_le_of_isStrongLimit' h₁ h₂
 
 /-- Given that `#α = #μ`, there are exactly `μ` Enumerations. -/
 @[simp]
 theorem mk_enumeration (mk_α : #α = #μ) : #(Enumeration α) = #μ := by
   refine le_antisymm ?_ ?_
   · rw [Cardinal.mk_congr enumerationEquiv]
-    simp only [mk_sigma, mk_pi, mk_α, prod_const, lift_id]
+    simp only [mk_sigma, mk_pi, mk_α, prod_const, Cardinal.lift_id]
     refine le_trans (sum_le_sum _ (fun _ => #μ) ?_) ?_
     · intro i
-      refine pow_le_of_isEtrongLimit Params.μ_isStrongLimit ?_
+      refine pow_le_of_isStrongLimit Params.μ_isStrongLimit ?_
       refine lt_of_lt_of_le ?_ Params.κ_le_μ_ord_cof
       exact card_typein_lt (· < ·) i Params.κ_ord.symm
-    · simp only [sum_const, lift_id, mul_mk_eq_max, ge_iff_le, max_le_iff, le_refl, and_true]
+    · simp only [sum_const, Cardinal.lift_id, mul_mk_eq_max, ge_iff_le, max_le_iff, le_refl,
+        and_true]
       exact Params.κ_lt_μ.le
   · rw [← mk_α]
     refine ⟨⟨fun x => ⟨1, fun _ _ => x⟩, ?_⟩⟩
@@ -287,6 +292,13 @@ theorem add_coe (E F : Enumeration α) :
     · refine ⟨E.max + i, add_lt_add_left hi E.max, ?_⟩
       rw [add_f_right_add]
 
+@[simp]
+theorem mem_add_iff (E F : Enumeration α) (x : α) :
+    x ∈ E + F ↔ x ∈ E ∨ x ∈ F := by
+  change x ∈ (E + F : Set α) ↔ _
+  rw [add_coe]
+  rfl
+
 @[simp]
 theorem smul_add {G : Type _} [SMul G α] {g : G} (E F : Enumeration α) :
     g • (E + F) = g • E + g • F := by
@@ -301,9 +313,100 @@ theorem smul_add {G : Type _} [SMul G α] {g : G} (E F : Enumeration α) :
         (show (g • E).max ≤ i from le_of_not_lt hi'), smul_f]
     rfl
 
-noncomputable section choose
+instance : LE (Enumeration α) where
+  le E F := E.max ≤ F.max ∧ ∀ (i : κ) (hE : i < E.max) (hF : i < F.max), E.f i hE = F.f i hF
+
+instance : LT (Enumeration α) where
+  lt E F := E.max < F.max ∧ ∀ (i : κ) (hE : i < E.max) (hF : i < F.max), E.f i hE = F.f i hF
+
+theorem le_iff (E F : Enumeration α) :
+    E ≤ F ↔ E.max ≤ F.max ∧ ∀ (i : κ) (hE : i < E.max) (hF : i < F.max), E.f i hE = F.f i hF :=
+  Iff.rfl
+
+theorem lt_iff (E F : Enumeration α) :
+    E < F ↔ E.max < F.max ∧ ∀ (i : κ) (hE : i < E.max) (hF : i < F.max), E.f i hE = F.f i hF :=
+  Iff.rfl
+
+instance : PartialOrder (Enumeration α) where
+  lt_iff_le_not_le E F := by
+    rw [le_iff, le_iff, lt_iff]
+    constructor
+    · intro h
+      exact ⟨⟨h.1.le, h.2⟩, fun h' => h'.1.not_lt h.1⟩
+    · intro h
+      rw [not_and'] at h
+      exact ⟨lt_of_le_not_le h.1.1 (h.2 (fun i hE hF => (h.1.2 i hF hE).symm)), h.1.2⟩
+  le_refl E := ⟨le_rfl, fun _ _ _ => rfl⟩
+  le_trans E F G hEF hFG := ⟨hEF.1.trans hFG.1, fun i hE hG =>
+    (hEF.2 i hE (hE.trans_le hEF.1)).trans (hFG.2 i (hE.trans_le hEF.1) hG)⟩
+  le_antisymm := by
+    rintro ⟨m₁, f₁⟩ ⟨m₂, f₂⟩ h₁₂ h₂₁
+    cases le_antisymm h₁₂.1 h₂₁.1
+    refine Enumeration.ext _ _ rfl (heq_of_eq ?_)
+    ext i hi
+    exact h₁₂.2 i hi hi
+
+theorem le_add (E F : Enumeration α) : E ≤ E + F := by
+  constructor
+  · simp only [add_max, le_add_iff_nonneg_right]
+    exact κ_pos _
+  · intro i hE hF
+    rw [add_f_left]
+
+noncomputable section
 open scoped Classical
 
+theorem ord_lt_of_small {s : Set α} (hs : Small s) [LinearOrder s] [IsWellOrder s (· < ·)] :
+    type ((· < ·) : s → s → Prop) < type ((· < ·) : κ → κ → Prop) := by
+  by_contra! h
+  have := card_le_card h
+  simp only [card_type] at this
+  exact hs.not_le this
+
+theorem typein_lt_type_of_small {s : Set α} (hs : Small s) [LinearOrder s] [IsWellOrder s (· < ·)]
+    {i : κ} (hi : i < enum (· < ·) (type ((· < ·) : s → s → Prop)) (ord_lt_of_small hs)) :
+    typein ((· < ·) : κ → κ → Prop) i < type ((· < ·) : s → s → Prop) := by
+  rw [← typein_lt_typein (· < ·), typein_enum] at hi
+  exact hi
+
+def ofSet' (s : Set α) (hs : Small s) [LinearOrder s] [IsWellOrder s (· < ·)] : Enumeration α where
+  max := enum (· < ·) (type ((· < ·) : s → s → Prop)) (ord_lt_of_small hs)
+  f i hi := (enum ((· < ·) : s → s → Prop) (typein ((· < ·) : κ → κ → Prop) i)
+    (typein_lt_type_of_small hs hi) : s)
+
+@[simp]
+theorem ofSet'_coe (s : Set α) (hs : Small s) [LinearOrder s] [IsWellOrder s (· < ·)] :
+    (ofSet' s hs : Set α) = s := by
+  ext x
+  rw [ofSet', mem_carrier_iff]
+  constructor
+  · rintro ⟨i, hi, rfl⟩
+    exact Subtype.coe_prop _
+  · rintro hx
+    refine ⟨enum ((· < ·) : κ → κ → Prop) (typein ((· < ·) : s → s → Prop) ⟨x, hx⟩) ?_, ?_, ?_⟩
+    · exact (typein_lt_type _ _).trans (ord_lt_of_small hs)
+    · rw [enum_lt_enum (r := ((· < ·) : κ → κ → Prop))]
+      exact typein_lt_type _ _
+    · simp only [typein_enum, enum_typein]
+
+def ofSet (s : Set α) (hs : Small s) : Enumeration α :=
+  letI := (IsWellOrder.subtype_nonempty (σ := s)).some.prop
+  letI := linearOrderOfSTO (IsWellOrder.subtype_nonempty (σ := s)).some.val
+  ofSet' s hs
+
+@[simp]
+theorem ofSet_coe (s : Set α) (hs : Small s) :
+    (ofSet s hs : Set α) = s :=
+  letI := (IsWellOrder.subtype_nonempty (σ := s)).some.prop
+  letI := linearOrderOfSTO (IsWellOrder.subtype_nonempty (σ := s)).some.val
+  ofSet'_coe s hs
+
+@[simp]
+theorem mem_ofSet_iff (s : Set α) (hs : Small s) (x : α) :
+    x ∈ ofSet s hs ↔ x ∈ s := by
+  change x ∈ (ofSet s hs : Set α) ↔ x ∈ s
+  rw [ofSet_coe]
+
 def chooseIndex (E : Enumeration α) (p : α → Prop)
     (h : ∃ i : κ, ∃ h : i < E.max, p (E.f i h)) : κ :=
   h.choose
@@ -316,7 +419,7 @@ theorem chooseIndex_spec {E : Enumeration α} {p : α → Prop}
     (h : ∃ i : κ, ∃ h : i < E.max, p (E.f i h)) : p (E.f (E.chooseIndex p h) (chooseIndex_lt h)) :=
   h.choose_spec.choose_spec
 
-end choose
+end
 
 end Enumeration
 
diff --git a/ConNF/Structural/Support.lean b/ConNF/Structural/Support.lean
index c1f47fd7c0..edde97c5b8 100644
--- a/ConNF/Structural/Support.lean
+++ b/ConNF/Structural/Support.lean
@@ -263,4 +263,99 @@ theorem mk_support : #(Support α) = #μ := by
     simp only [SupportCondition.mk.injEq, inl.injEq, true_and] at this
     exact this
 
+/-- `S` is a *completion* of an enumeration of support conditions `E` if it extends `E`,
+and every support condition in the extension is an atom contained in the symmetric difference of
+two near-litters in `E`. -/
+structure IsCompletion (S : Support α) (E : Enumeration (SupportCondition α)) : Prop where
+  le : E ≤ S.enum
+  eq_atom (i : κ) (hi₁ : i < S.max) (hi₂ : E.max ≤ i) :
+    ∃ A : ExtendedIndex α, ∃ a : Atom, ∃ N₁ N₂ : NearLitter,
+    N₁.1 = N₂.1 ∧ a ∈ (N₁ : Set Atom) ∆ N₂ ∧
+    ⟨A, inr N₁⟩ ∈ E ∧ ⟨A, inr N₂⟩ ∈ E ∧ S.f i hi₁ = ⟨A, inl a⟩
+
+/-- The set of support conditions that we need to add to `s` to make it a support. -/
+def completionToAdd (s : Set (SupportCondition α)) : Set (SupportCondition α) :=
+  {x | ∃ N₁ N₂ : NearLitter, N₁.1 = N₂.1 ∧ ∃ a : Atom, x.2 = inl a ∧ a ∈ (N₁ : Set Atom) ∆ N₂ ∧
+    ⟨x.1, inr N₁⟩ ∈ s ∧ ⟨x.1, inr N₂⟩ ∈ s}
+
+theorem nearLitter_not_mem_completionToAdd (A : ExtendedIndex α) (N : NearLitter)
+    (s : Set (SupportCondition α)) : ⟨A, inr N⟩ ∉ completionToAdd s := by
+  rintro ⟨_, _, _, a, h, _⟩
+  cases h
+
+def completionToAdd' (s : Set (SupportCondition α)) (A : ExtendedIndex α) : Set Atom :=
+  ⋃ (N₁ : NearLitter) (_ : N₁ ∈ (fun N => ⟨A, inr N⟩) ⁻¹' s)
+    (N₂ : NearLitter) (_ : N₂ ∈ (fun N => ⟨A, inr N⟩) ⁻¹' s)
+    (_ : N₁.1 = N₂.1),
+  (N₁ : Set Atom) ∆ N₂
+
+theorem completionToAdd'_small (s : Set (SupportCondition α)) (hs : Small s) (A : ExtendedIndex α) :
+    Small (completionToAdd' s A) := by
+  have : Function.Injective (fun N => (⟨A, inr N⟩ : SupportCondition α))
+  · intro N₁ N₂ h
+    simp only [SupportCondition.mk.injEq, inr.injEq, true_and] at h
+    exact h
+  refine Small.bUnion (Small.preimage this hs) (fun N₁ _ => ?_)
+  refine Small.bUnion (Small.preimage this hs) (fun N₂ _ => ?_)
+  refine small_iUnion_Prop (fun h => ?_)
+  refine N₁.2.prop.symm.trans ?_
+  rw [h]
+  exact N₂.2.prop
+
+theorem completionToAdd_eq_completionToAdd' (s : Set (SupportCondition α)) :
+    completionToAdd s = ⋃ (A : ExtendedIndex α), (⟨A, inl ·⟩) '' completionToAdd' s A := by
+  simp only [completionToAdd, completionToAdd']
+  aesop
+
+theorem completionToAdd_small (s : Set (SupportCondition α)) (hs : Small s) :
+    Small (completionToAdd s) := by
+  rw [completionToAdd_eq_completionToAdd']
+  refine small_iUnion ?_ ?_
+  · exact (mk_extendedIndex α).trans_lt Params.Λ_lt_κ
+  · intro A
+    exact Small.image (completionToAdd'_small s hs A)
+
+noncomputable def completeEnum (E : Enumeration (SupportCondition α)) :
+    Enumeration (SupportCondition α) :=
+  E + Enumeration.ofSet (completionToAdd E) (completionToAdd_small _ E.small)
+
+@[simp]
+theorem mem_completeEnum (E : Enumeration (SupportCondition α)) (c : SupportCondition α) :
+    c ∈ completeEnum E ↔ c ∈ E ∨ c ∈ completionToAdd E :=
+  by rw [completeEnum, Enumeration.mem_add_iff, Enumeration.mem_ofSet_iff]
+
+theorem completeEnum_mem_of_mem_symmDiff (E : Enumeration (SupportCondition α))
+    (A : ExtendedIndex α) (N₁ N₂ : NearLitter) (a : Atom) :
+    N₁.1 = N₂.1 → a ∈ (N₁ : Set Atom) ∆ N₂ →
+    ⟨A, inr N₁⟩ ∈ completeEnum E → ⟨A, inr N₂⟩ ∈ completeEnum E → ⟨A, inl a⟩ ∈ completeEnum E := by
+  intro hN ha hN₁ hN₂
+  rw [mem_completeEnum] at hN₁ hN₂ ⊢
+  obtain (hN₁ | hN₁) := hN₁
+  · obtain (hN₂ | hN₂) := hN₂
+    · exact Or.inr ⟨N₁, N₂, hN, a, rfl, ha, hN₁, hN₂⟩
+    · cases nearLitter_not_mem_completionToAdd A N₂ _ hN₂
+  · cases nearLitter_not_mem_completionToAdd A N₁ _ hN₁
+
+/-- Extend an enumeration to a support. -/
+noncomputable def complete (E : Enumeration (SupportCondition α)) : Support α where
+  enum := completeEnum E
+  mem_of_mem_symmDiff' := completeEnum_mem_of_mem_symmDiff E
+
+theorem complete_isCompletion (E : Enumeration (SupportCondition α)) :
+    IsCompletion (complete E) E := by
+  constructor
+  · exact Enumeration.le_add _ _
+  · intro i hi₁ hi₂
+    have := Enumeration.f_mem _ (i - E.max) (κ_sub_lt hi₁ hi₂)
+    rw [← Enumeration.add_f_right hi₁ hi₂, Enumeration.mem_ofSet_iff] at this
+    obtain ⟨N₁, N₂, hN, a, ha₁, ha₂, hN₁, hN₂⟩ := this
+    refine ⟨_, a, N₁, N₂, hN, ha₂, hN₁, hN₂, ?_⟩
+    rw [← ha₁]
+    rfl
+
+/-- `S` is a *sum* of `S₁` and `S₂` if it is a completion of `S₁ + S₂`. -/
+def IsSum (S S₁ S₂ : Support α) : Prop := IsCompletion S (S₁.enum + S₂.enum)
+
+theorem exists_isSum (S₁ S₂ : Support α) : ∃ S, IsSum S S₁ S₂ := ⟨_, complete_isCompletion _⟩
+
 end ConNF