diff --git a/ConNF.lean b/ConNF.lean
index a0d579eed8..42691724f7 100644
--- a/ConNF.lean
+++ b/ConNF.lean
@@ -15,6 +15,7 @@ import ConNF.Construction.InductionStatement
 import ConNF.Construction.NewModelData
 import ConNF.Construction.RaiseStrong
 import ConNF.Construction.RunInduction
+import ConNF.Construction.TTT
 import ConNF.Counting.BaseCoding
 import ConNF.Counting.BaseCounting
 import ConNF.Counting.CodingFunction
diff --git a/ConNF/Construction/Code.lean b/ConNF/Construction/Code.lean
index 09e80bd527..5fb00d6763 100644
--- a/ConNF/Construction/Code.lean
+++ b/ConNF/Construction/Code.lean
@@ -27,6 +27,7 @@ class TypedNearLitters (α : Λ) [ModelData α] [Position (Tangle α)] where
 
 export TypedNearLitters (typed)
 
+@[ext]
 class LtData where
   [data : (β : TypeIndex) → [LtLevel β] → ModelData β]
   [positions : (β : TypeIndex) → [LtLevel β] → Position (Tangle β)]
diff --git a/ConNF/Construction/Externalise.lean b/ConNF/Construction/Externalise.lean
index d1e73502b4..758dfd96b3 100644
--- a/ConNF/Construction/Externalise.lean
+++ b/ConNF/Construction/Externalise.lean
@@ -42,6 +42,20 @@ theorem heq_funext {α : Sort _} {β γ : α → Sort _} {f : (x : α) → β x}
   simp only [heq_eq_eq] at h ⊢
   exact funext h
 
+theorem globalLtData_eq [Level] :
+    globalLtData = ltData (λ β _ ↦ motive β) := by
+  apply LtData.ext
+  · ext β hβ
+    induction β using recBotCoe
+    case bot => rfl
+    case coe β => rfl
+  · apply heq_funext
+    intro β
+    induction β using recBotCoe
+    case bot => rfl
+    case coe β => rfl
+  · rfl
+
 theorem globalLeData_eq [Level] :
     globalLeData = leData (λ β _ ↦ motive β) := by
   apply LeData.ext
diff --git a/ConNF/Construction/TTT.lean b/ConNF/Construction/TTT.lean
new file mode 100644
index 0000000000..a970fdbe72
--- /dev/null
+++ b/ConNF/Construction/TTT.lean
@@ -0,0 +1,104 @@
+import ConNF.Construction.Externalise
+
+/-!
+# New file
+
+In this file...
+
+## Main declarations
+
+* `ConNF.foo`: Something new.
+-/
+
+noncomputable section
+universe u
+
+open Cardinal Ordinal
+
+open scoped Pointwise
+
+namespace ConNF
+
+variable [Params.{u}]
+
+/-- A redefinition of the derivative of allowable permutations that is invariant of level,
+but still has nice definitional properties. -/
+@[default_instance 200]
+instance {β γ : TypeIndex} : Derivative (AllPerm β) (AllPerm γ) β γ where
+  deriv ρ A :=
+    A.recSderiv
+    (motive := λ (δ : TypeIndex) (A : β ↝ δ) ↦
+      letI : Level := ⟨δ.recBotCoe (Nonempty.some inferInstance) id⟩
+      letI : LeLevel δ := ⟨δ.recBotCoe (λ _ ↦ bot_le) (λ _ h ↦ WithBot.coe_le_coe.mpr h.le)
+        (show δ.recBotCoe (Nonempty.some inferInstance) id = Level.α from rfl)⟩
+      AllPerm δ)
+    ρ (λ δ ε A h ρ ↦
+      letI : Level := ⟨δ.recBotCoe (Nonempty.some inferInstance) id⟩
+      letI : LeLevel δ := ⟨δ.recBotCoe (λ _ ↦ bot_le) (λ _ h ↦ WithBot.coe_le_coe.mpr h.le)
+        (show δ.recBotCoe (Nonempty.some inferInstance) id = Level.α from rfl)⟩
+      letI : LeLevel ε := ⟨h.le.trans LeLevel.elim⟩
+      PreCoherentData.allPermSderiv h ρ)
+
+@[simp]
+theorem allPerm_deriv_nil' {β : TypeIndex}
+    (ρ : AllPerm β) :
+    ρ ⇘ (.nil : β ↝ β) = ρ :=
+  rfl
+
+@[simp]
+theorem allPerm_deriv_sderiv' {β γ δ : TypeIndex}
+    (ρ : AllPerm β) (A : β ↝ γ) (h : δ < γ) :
+    ρ ⇘ (A ↘ h) = ρ ⇘ A ↘ h :=
+  rfl
+
+def Symmetric {α β : Λ} (s : Set (TSet β)) (hβ : (β : TypeIndex) < α) : Prop :=
+  ∃ S : Support α, ∀ ρ : AllPerm α, ρᵁ • S = S → ρ ↘ hβ • s = s
+
+def newSetEquiv {α : Λ} :
+    letI : Level := ⟨α⟩
+    @TSet _ α newModelData.toPreModelData ≃ TSet α :=
+  letI : Level := ⟨α⟩
+  castTSet (D₁ := newModelData) (D₂ := globalModelData) rfl
+    (by rw [globalModelData, motive_eq, constructMotive, globalLtData_eq])
+
+def allPermEquiv {α : Λ} :
+    letI : Level := ⟨α⟩
+    NewPerm ≃ AllPerm α :=
+  letI : Level := ⟨α⟩
+  castAllPerm (D₁ := newModelData) (D₂ := globalModelData) rfl
+    (by rw [globalModelData, motive_eq, constructMotive, globalLtData_eq])
+
+@[simp]
+theorem allPermEquiv_forget {α : Λ} (ρ : letI : Level := ⟨α⟩; NewPerm) :
+    (allPermEquiv ρ)ᵁ = ρᵁ :=
+  letI : Level := ⟨α⟩
+  castAllPerm_forget (D₁ := newModelData) (D₂ := globalModelData) _ ρ
+
+theorem exists_of_symmetric {α β : Λ} (s : Set (TSet β)) (hβ : (β : TypeIndex) < α)
+    (hs : Symmetric s hβ) :
+    ∃ x : TSet α, ∀ y : TSet β, y ∈[hβ] x ↔ y ∈ s := by
+  letI : Level := ⟨α⟩
+  letI : LtLevel β := ⟨hβ⟩
+  suffices ∃ x : (@TSet _ α newModelData.toPreModelData), ∀ y : TSet β, yᵁ ∈[hβ] xᵁ ↔ y ∈ s by
+    obtain ⟨x, hx⟩ := this
+    use newSetEquiv x
+    intro y
+    rw [← hx]
+    sorry
+  obtain rfl | hs' := s.eq_empty_or_nonempty
+  · use none
+    intro y
+    simp only [Set.mem_empty_iff_false, iff_false]
+    exact not_mem_none y
+  · use some (Code.toSet ⟨β, s, hs'⟩ ?_)
+    · intro y
+      erw [mem_some_iff]
+      exact Code.mem_toSet _
+    · obtain ⟨S, hS⟩ := hs
+      use S
+      intro ρ hρS
+      have := hS (allPermEquiv ρ) ?_
+      · sorry
+      · rwa [allPermEquiv_forget]
+
+end ConNF