diff --git a/Cslib.lean b/Cslib.lean
index 34a0d27bef..762b404734 100644
--- a/Cslib.lean
+++ b/Cslib.lean
@@ -158,7 +158,9 @@ public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.LcAt
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.LeftmostReduction
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.MultiApp
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.MultiSubst
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.ParEtaC
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.Properties
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.Size
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.StandardReduction
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.StrongNorm
 public import Cslib.Languages.LambdaCalculus.Named.Untyped.Basic
diff --git a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/FullEta.lean b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/FullEta.lean
index 65df46418e..fb182598c5 100644
--- a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/FullEta.lean
+++ b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/FullEta.lean
@@ -7,8 +7,10 @@ Authors: Maximiliano Onofre Martínez
 module
 
 public import Cslib.Foundations.Relation.Attr
+public import Cslib.Foundations.Relation.Defs
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.Properties
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.Congruence
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.Size
 
 /-! # η-reduction for the λ-calculus -/
 
@@ -149,6 +151,13 @@ lemma steps_open_cong_r {s t t' : Term Var} (lc_s : LC s.abs) (steps : t ↠η
   case refl => rfl
   case head _ _ st _ ih => exact .trans (step_open_cong_r lc_s st) ih
 
+lemma step_size (step : M ⭢ηᶠ M') : M'.size < M.size := by
+  induction step with
+  | abs xs _ =>
+    have ⟨x, _⟩ := fresh_exists <| free_union [fv] Var
+    grind
+  | _ => grind
+
 /- Closing a sequence of η-reduction steps over a fresh variable preserves the steps. -/
 open Relation in
 lemma close_eta_steps (hx_M : x ∉ M.fv) (st_M : ReflGen FullEta (M ^ fvar x) N) :
@@ -158,6 +167,16 @@ lemma close_eta_steps (hx_M : x ∉ M.fv) (st_M : ReflGen FullEta (M ^ fvar x) N
   | single st =>
     exact .single (Xi.abs {x} (by grind [step_subst_cong_l]))
 
+open Relation in
+lemma terminating : Terminating (@FullEta Var) := by
+  apply WellFounded.intro
+  intros t
+  induction h : t.size using Nat.strong_induction_on generalizing t with | h n ih =>
+  apply Acc.intro
+  intros y hy
+  apply FullEta.step_size at hy
+  exact ih y.size (by omega) _ rfl
+
 end LambdaCalculus.LocallyNameless.Untyped.Term.FullEta
 
 end Cslib
diff --git a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/ParEtaC.lean b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/ParEtaC.lean
new file mode 100644
index 0000000000..bf65a8b4e4
--- /dev/null
+++ b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/ParEtaC.lean
@@ -0,0 +1,314 @@
+/-
+Copyright (c) 2025 Chris Henson. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Yijun Leng
+-/
+
+
+module
+
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.FullBeta
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.FullEta
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.Size
+
+/-!
+# η-expansion preserves β-strong-normalisation (`sn_eta_step`)
+
+This file formalises the paper proof in `sn_eta_step_proof.md`:
+
+  **If `t ⟶η t'` (a single full η-step) and `t'` is β-strongly-normalising,
+  then `t` is β-strongly-normalising.**
+
+("β-strongly-normalising" is `Acc (flip FullBeta)`, i.e. accessibility for single
+β-steps.)
+
+The proof follows Takahashi-style **parallel η-reduction with an explicit
+`Eta`-count**.  We define `ParEtaC n M N` = "`M` reduces to `N` by a parallel
+η-derivation containing exactly `n` contractions of an η-redex".  The three
+ingredients are:
+
+* `parEtaC_of_fullEta` (Fact 2.1): a single η-step gives a count-`1` derivation;
+* `interaction` (the Interaction Lemma): a single β-step out of `t` either
+  reflects to a genuine β-step out of `t'` (keeping some parallel η-derivation),
+  or is *absorbed*, landing back on `t'` with a **strictly smaller** count;
+* `sn_transfer` (the generalized SN-transfer theorem): lexicographic induction on
+  `(β-accessibility rank of t', count n)`.
+-/
+
+@[expose] public section
+
+set_option linter.unusedDecidableInType false
+
+namespace Cslib
+
+universe u
+
+namespace LambdaCalculus.LocallyNameless.Untyped.Term
+
+variable {Var : Type u}
+
+/-- **Parallel η-reduction with `Eta`-count** `ParEtaC n M N`: `M` reduces to `N`
+by a parallel η-derivation whose number of η-contractions is exactly `n`. -/
+inductive ParEtaC : ℕ → Term Var → Term Var → Prop
+  /-- (η1) A free variable reduces to itself, count `0`. -/
+  | fvar (x : Var) : ParEtaC 0 (Term.fvar x) (Term.fvar x)
+  /-- (η3) Congruence for application; counts add. -/
+  | app {a b : ℕ} {M M' N N' : Term Var} :
+      ParEtaC a M M' → ParEtaC b N N' → ParEtaC (a + b) (Term.app M N) (Term.app M' N')
+  /-- (η2) Congruence for abstraction (cofinite quantification); count preserved. -/
+  | abs (xs : Finset Var) {a : ℕ} {M M' : Term Var} :
+      (∀ x ∉ xs, ParEtaC a (M ^ Term.fvar x) (M' ^ Term.fvar x)) →
+        ParEtaC a (Term.abs M) (Term.abs M')
+  /-- (η4) Parallel contraction of an η-redex `λz.(M z) ⟹ M'`; count `+1`. -/
+  | eta {a : ℕ} {M M' : Term Var} :
+      LC M → ParEtaC a M M' → ParEtaC (a + 1) (Term.abs (Term.app M (Term.bvar 0))) M'
+
+/-- `ParEtaC` is reflexive at count `0` on locally closed terms. -/
+theorem ParEtaC.refl {M : Term Var} (h : LC M) : ParEtaC 0 M M := by
+  induction n : M.size using Nat.strong_induction_on generalizing M with | h n ih =>
+  match h with
+  | LC.fvar x => exact ParEtaC.fvar x
+  | LC.app h₁ h₂ => exact ParEtaC.app (ih _ (by grind) h₁ rfl) (ih _ (by grind) h₂ rfl)
+  | LC.abs xs t ht =>
+      exact ParEtaC.abs xs (fun x hx => ih _ (by rw [size_open_fvar]; grind) (ht x hx) rfl)
+
+/-- A single full η-step is a parallel η-derivation of count `1`. -/
+theorem parEtaC_of_fullEta {t t' : Term Var} (h : FullEta t t') : ParEtaC 1 t t' := by
+  induction h with
+  | base hEta => cases hEta with | eta hLC => exact ParEtaC.eta hLC (ParEtaC.refl hLC)
+  | appL hLC _ hih => exact ParEtaC.app (ParEtaC.refl hLC) hih
+  | appR hLC _ hih => exact ParEtaC.app hih (ParEtaC.refl hLC)
+  | abs k _ ih => exact ParEtaC.abs k ih
+
+@[scoped grind →]
+theorem ParEtaC.step_lc_r {n : ℕ} {M N : Term Var} (h : ParEtaC n M N) : LC N := by
+  induction h with
+  | fvar x => exact LC.fvar x
+  | app _ _ ihM ihN => exact LC.app ihM ihN
+  | abs xs _ ih => exact LC.abs xs _ (fun x hx => (ih x hx))
+  | eta _ _ ih => exact ih
+
+variable [HasFresh Var]
+
+@[scoped grind →]
+theorem ParEtaC.step_lc_l {n : ℕ} {M N : Term Var} (h : ParEtaC n M N) : LC M := by
+  induction h with
+  | fvar x => exact LC.fvar x
+  | app _ _ ihM ihN => exact LC.app ihM ihN
+  | abs xs _ ih => exact LC.abs xs _ (fun x hx => (ih x hx))
+  | eta hM hMM' ih => exact LC.abs (∅ : Finset Var) _ (fun y _ => LC.app (by grind) (by grind))
+
+variable [DecidableEq Var]
+
+theorem ParEtaC.refl_rev {M N : Term Var} (h : ParEtaC 0 M N) : M = N := by
+  generalize hi: 0 = i at h
+  induction h with
+  | fvar x => grind
+  | app _ _ _ _ => grind
+  | eta _ _ _ => grind
+  | abs xs _ ih =>  have ⟨x, _⟩ := fresh_exists <| free_union [fv] Var
+                    specialize ih x (by grind) hi
+                    apply open_injective at ih <;> grind
+
+theorem parEtaC_to_fullEta {t t' : Term Var} (h : ParEtaC 1 t t') : FullEta t t' := by
+  generalize hi: 1 = i at h
+  induction h with
+  | fvar x => omega
+  | abs xs _ _ => exact Xi.abs xs (by grind)
+  | app ha hb iha ihb =>
+    rename_i a b _ _ _ _
+    have hab : (a = 0 /\ b = 1) \/ (a = 1 /\ b = 0) := by omega
+    rcases hab with ⟨ha, hb⟩|⟨ha, hb⟩
+    all_goals subst_vars
+    · have := ParEtaC.refl_rev ha
+      subst_vars
+      exact Xi.appL (ParEtaC.step_lc_r ha) (by grind)
+    · have := ParEtaC.refl_rev hb
+      subst_vars
+      exact Xi.appR (ParEtaC.step_lc_r hb) (by grind)
+  | eta hlc h ih => cases hi
+                    apply ParEtaC.refl_rev at h
+                    grind
+
+/-- **Renaming preserves the count.** Substituting one free variable for another
+in a `ParEtaC` derivation preserves the derivation and its count. -/
+theorem ParEtaC.subst {n : ℕ} {A B : Term Var} (h : ParEtaC n A B) (x y : Var) :
+    ParEtaC n A[x:=Term.fvar y] B[x:=Term.fvar y] := by
+  induction h with
+  | fvar z => rw [subst_fvar]
+              split <;> apply ParEtaC.refl (LC.fvar _)
+  | eta hM hMM' ih => exact ParEtaC.eta (subst_lc hM (LC.fvar y)) ih
+  | app hM hN ihM ihN => exact ParEtaC.app ihM ihN
+  | @abs xs a M M' hbody ih =>
+      refine ParEtaC.abs (xs ∪ {x}) (fun z hz => ?_)
+      have key := ih z (by grind)
+      rw [subst_open_var, subst_open_var] at key <;> grind
+
+/-- Build an abstraction derivation from a single fresh-variable body instance. -/
+theorem ParEtaC.abs_of_open {m : ℕ} {N s' : Term Var} (x : Var)
+    (hx : x ∉ fv N) (hx' : x ∉ fv s') (h : ParEtaC m (N ^ Term.fvar x) (s' ^ Term.fvar x)) :
+    ParEtaC m (Term.abs N) (Term.abs s') := by
+  refine ParEtaC.abs (fv N ∪ fv s') ?_
+  intro y hy
+  by_cases hyc : y = x
+  · rw [hyc]; exact h
+  · grind [ParEtaC.subst h x y]
+
+/-- **(Substitutivity)** If `M ⟹η M'` and `N ⟹η N'`, then
+`M[x:=N] ⟹η M'[x:=N']` (for some count `c`). -/
+theorem ParEtaC.substC {a b : ℕ} {M M' N N' : Term Var} (x : Var)
+    (hM : ParEtaC a M M') (hN : ParEtaC b N N') :
+    ∃ c, ParEtaC c M[x:=N] M'[x:=N'] := by
+  induction hM generalizing N N' with
+  | fvar y =>
+      rw [subst_fvar, subst_fvar]
+      split
+      · grind
+      · exact ⟨0, ParEtaC.fvar y⟩
+  | app hM hN ihM ihN =>
+      obtain ⟨c1, hc1⟩ := ihM hN
+      obtain ⟨c2, hc2⟩ := ihN hN
+      exact ⟨c1 + c2, ParEtaC.app hc1 hc2⟩
+  | @abs xs a M M' hbody ih =>
+      have ⟨y, hy⟩ := fresh_exists <| free_union [fv] Var
+      obtain ⟨c, hc⟩ := ih y (by grind) hN
+      refine ⟨c, abs_of_open y ?_ ?_ ?_⟩
+      · grind [subst_preserve_not_fvar]
+      · grind [subst_preserve_not_fvar]
+      · grind [ParEtaC.step_lc_l, ParEtaC.step_lc_r]
+  | @eta a M M' hM hMM' ih =>
+      obtain ⟨c, hc⟩ := ih hN
+      exact ⟨c + 1, ParEtaC.eta hc.step_lc_l hc⟩
+
+/-- Opening form of substitutivity: from `abs M ⟹η abs M'` and `N ⟹η N'`, the
+opened bodies satisfy `M^N ⟹η M'^N'` (for some count). -/
+theorem ParEtaC.open_of_absBody {a b : ℕ} (xs : Finset Var) {M M' N N' : Term Var}
+    (hbody : ∀ x ∉ xs, ParEtaC a (M ^ Term.fvar x) (M' ^ Term.fvar x))
+    (hN : ParEtaC b N N') :
+    ∃ c, ParEtaC c (M ^ N) (M' ^ N') := by
+  have ⟨x, hx⟩ := fresh_exists <| free_union [fv] Var
+  obtain ⟨c, hc⟩ := ParEtaC.substC x (hbody x (by grind)) hN
+  exact ⟨c, by grind⟩
+
+omit [HasFresh Var] in
+/-- Opening by a fresh free variable is injective. -/
+theorem open_fvar_inj {A B : Term Var} {x : Var} (hA : x ∉ fv A) (hB : x ∉ fv B)
+    (h : A ^ Term.fvar x = B ^ Term.fvar x) : A = B := by
+  have hcl : (A ^ Term.fvar x) ^* x = (B ^ Term.fvar x) ^* x := by rw [h]
+  rw [<- open_close_var, <- open_close_var] at hcl
+  all_goals grind
+
+theorem ParEtaC.toFullEtaStar {i : Nat} {M N : Term Var} (h : ParEtaC i M N) : M ↠ηᶠ N := by
+  induction h with
+  | eta hM hMM' ih => exact .head (Xi.base (.eta hM)) ih
+  | fvar x => exact Relation.ReflTransGen.refl
+  | abs xs h ih => exact FullEta.redex_abs_cong xs ih
+  | app hM hN ihM ihN =>
+      exact Relation.ReflTransGen.trans (FullEta.redex_app_l_cong ihM ((ParEtaC.step_lc_l hN)))
+                                        (FullEta.redex_app_r_cong ihN ((ParEtaC.step_lc_r hM)))
+
+
+/-- The conclusion of the Interaction Lemma at a fixed source term `t`. -/
+def InteractionAt (t : Term Var) : Prop :=
+  ∀ (n : ℕ) (t' s : Term Var), ParEtaC n t t' → FullBeta t s →
+    (∃ s' m, ParEtaC m s s' ∧ FullBeta t' s') ∨ (∃ m, m < n ∧ ParEtaC m s t')
+
+/-- **The Interaction Lemma.** A single β-step `t ⟶β s` against a parallel
+η-derivation `t ⟹η t'` either reflects to a genuine β-step `t' ⟶β s'` (with `s`
+still parallel-η-reducing to `s'`), or is absorbed — landing back on `t'` with a
+strictly smaller η-count. -/
+theorem interaction (t : Term Var) : InteractionAt t := by
+  induction heq : t.size using Nat.strong_induction_on generalizing t with | h n IH' =>
+  have IH : ∀ t : Term Var, t.size < n → t.InteractionAt := fun t h =>
+    IH' _ (Nat.lt_of_lt_of_eq (by omega) (by rfl)) _ rfl
+  clear IH'
+  subst n
+  intro n t' s hp hb
+  cases hb with
+  | base hβ =>
+    -- redex at the top: t = app (abs M) N, s = M ^ N
+    cases hβ with
+    | @beta M N hM hN =>
+      cases hp with
+      | @app a b _ F _ N' hf hn =>
+        cases hf with
+        | abs ys hbody =>
+          -- t' = app (abs M') N' still a redex; genuine β-step on t'
+          obtain ⟨c, hc⟩ := ParEtaC.open_of_absBody ys hbody hn
+          exact Or.inl ⟨_, c, hc,
+            Xi.base (Beta.beta (ParEtaC.step_lc_r (ParEtaC.abs ys hbody)) (ParEtaC.step_lc_r hn))⟩
+        | @eta a2 P _ hP hPF =>
+          -- M = app P (bvar 0); the β-step is absorbed by the η-redex
+          refine Or.inr ⟨a2 + b, by omega, ParEtaC.app ?_ hn⟩
+          rw [open_lc _ _ _ hP]
+          exact hPF
+  | @appL Z M0 N0 hZ hstep =>
+    cases hp with
+    | @app a b _ Z' _ M0' hZ' hM0 =>
+      rcases IH M0 (by grind) _ _ _ hM0 hstep with ⟨s'', m, hpar, hbeta⟩ | ⟨m, hm, hpar⟩
+      · exact Or.inl ⟨_, a + m, ParEtaC.app hZ' hpar, Xi.appL (ParEtaC.step_lc_r hZ') hbeta⟩
+      · exact Or.inr ⟨a + m, by omega, ParEtaC.app hZ' hpar⟩
+  | @appR Z M0 N0 hZ hstep =>
+    cases hp with
+    | @app a b M0' _ _ Z' hM0 hZ' =>
+      rcases IH M0 (by grind) _ _ _ hM0 hstep with ⟨s'', m, hpar, hbeta⟩ | ⟨m, hm, hpar⟩
+      · exact Or.inl ⟨_, m + b, ParEtaC.app hpar hZ', Xi.appR (ParEtaC.step_lc_r hZ') hbeta⟩
+      · exact Or.inr ⟨m + b, by omega, ParEtaC.app hpar hZ'⟩
+  | @abs M0 N0 xs hbodystep =>
+    cases hp with
+    | @abs _ _ ys M0' hbody =>
+      -- t' = abs M0', hbody : ∀ x ∉ ys, ParEtaC n (M0^x) (M0'^x)
+      have ⟨x, hx⟩ := fresh_exists <| free_union [fv] Var
+      have hsz : size (M0 ^ Term.fvar x) < size M0.abs := by
+        rw [size_open_fvar]
+        grind
+      rcases IH (M0 ^ Term.fvar x) hsz _ _ _ (hbody x (by grind)) (hbodystep x (by grind)) with
+        ⟨s'', m, hpar, hbeta⟩ | ⟨m, hm, hpar⟩
+      · refine Or.inl ⟨(s'' ^* x).abs, m, ParEtaC.abs_of_open x (by grind) (by grind) ?_, ?_⟩
+        · rw [close_open _ _ (FullBeta.step_lc_r hbeta)]
+          grind
+        · rw [open_close x M0' 0 (by grind)]
+          exact FullBeta.step_abs_close (by grind)
+      · exact Or.inr ⟨m, hm, ParEtaC.abs_of_open x (by grind) (by grind) hpar⟩
+    | @eta a2 P _ hP hPF =>
+      -- M0 = app P (bvar 0), n = a2 + 1, hPF : ParEtaC a2 P t'
+      have ⟨x, hx⟩ := fresh_exists <| free_union [fv] Var
+      have hstepx : FullBeta (Term.app P (Term.fvar x)) (N0 ^ Term.fvar x) := by
+        grind [hbodystep x (by grind)]
+      generalize hw : N0 ^ Term.fvar x = w at hstepx
+      cases hstepx with
+      | base hβ =>
+        cases hβ with
+        | @beta Q Narg hQ hNarg =>
+          -- P = abs Q, Narg = fvar x, w = Q ^ fvar x, hw : N0^x = Q^x
+          have hN0Q : N0 = Q := open_fvar_inj (by grind) (by grind) hw
+          exact Or.inr ⟨a2, by omega, by rw [hN0Q]; exact hPF⟩
+      | @appL Z M N hZ hxi =>
+        cases hxi with | base hb2 => cases hb2
+      | @appR Z M N hZ hxi =>
+        -- M = P, Z = fvar x; hxi : FullBeta P N; w = app N (fvar x)
+        have hPsLC : LC N := FullBeta.step_lc_r hxi
+        have hxN : x ∉ fv N := by grind [FullBeta.step_not_fv hxi]
+        have hNe : N0 = Term.app N (Term.bvar 0) := by apply @open_fvar_inj _ _ _ _ x <;> grind
+        rcases IH P (by grind) _ _ _ hPF hxi with ⟨s', m, hpar, hbeta⟩ | ⟨m, hm, hpar⟩
+        · exact Or.inl ⟨s', m + 1, by rw [hNe]; exact ParEtaC.eta hPsLC hpar, hbeta⟩
+        · exact Or.inr ⟨m + 1, by omega, by rw [hNe]; exact ParEtaC.eta hPsLC hpar⟩
+
+/-- **Generalized SN-transfer theorem.**  If `t ⟹η t'` (parallel η, any count)
+and `t'` is β-strongly-normalising, then so is `t`. -/
+theorem sn_transfer {t t' : Term Var}
+    (hacc : Relation.SN (FullBeta : Term Var → Term Var → Prop) t')
+    {n : ℕ} (hp : ParEtaC n t t') :
+    Relation.SN (FullBeta : Term Var → Term Var → Prop) t := by
+  induction hacc generalizing t n with
+  | intro c hc ih =>
+    induction n using Nat.strong_induction_on generalizing t with
+    | _ n ihn =>
+      refine Acc.intro t (fun s hs => ?_)
+      rcases interaction _ _ _ _ hp hs with ⟨s', m, hps', hb'⟩ | ⟨m, hm, hps⟩
+      · exact ih s' hb' hps'
+      · exact ihn m hm hps
+
+end LambdaCalculus.LocallyNameless.Untyped.Term
+
+end Cslib
diff --git a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/Properties.lean b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/Properties.lean
index 4af9462d16..c58b01f733 100644
--- a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/Properties.lean
+++ b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/Properties.lean
@@ -151,9 +151,13 @@ lemma close_open_to_subst (m n : Term Var) (x : Var) (m_lc : LC m) (n_lc : LC n)
     (m ^* x) ^ n = m[x := n] := close_openRec_to_subst m n x 0 m_lc n_lc
 
 /-- Closing and opening are inverses. -/
-lemma close_open (x : Var) (t : Term Var) (k : ℕ) (t_lc : LC t) : t⟦k ↜ x⟧⟦k ↝ fvar x⟧ = t := by
+lemma close_openRec (x : Var) (t : Term Var) (k : ℕ) (t_lc : LC t) : t⟦k ↜ x⟧⟦k ↝ fvar x⟧ = t := by
   grind [subst_refl]
 
+/-- Closing and opening are inverses. -/
+lemma close_open (x : Var) (t : Term Var) (t_lc : LC t) : (t ^* x) ^ fvar x = t :=
+  close_openRec x t 0 t_lc
+
 end LambdaCalculus.LocallyNameless.Untyped.Term
 
 end Cslib
diff --git a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/Size.lean b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/Size.lean
new file mode 100644
index 0000000000..e1313ec340
--- /dev/null
+++ b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/Size.lean
@@ -0,0 +1,40 @@
+/-
+Copyright (c) 2025 Chris Henson. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Yijun Leng
+-/
+
+module
+
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.Basic
+
+/-! Size of untyped lambda calculus term. -/
+
+@[expose] public section
+
+namespace Cslib
+
+namespace LambdaCalculus.LocallyNameless.Untyped.Term
+
+universe u
+
+variable {Var : Type u}
+
+/-- Computes the size of a lambda calculus term. -/
+@[simp, scoped grind =]
+def size : Term Var -> Nat
+| bvar _ => 0
+| fvar _ => 0
+| abs t => 1 + size t
+| app t1 t2 => 1 + size t1 + size t2
+
+@[scoped grind =]
+theorem size_openRec_fvar (i) (x : Var) (M) : M⟦i ↝ fvar x⟧.size = M.size := by
+  induction M generalizing i <;> grind
+
+theorem size_open_fvar (x : Var) (t : Term Var) : size (t ^ fvar x) = size t :=
+  size_openRec_fvar 0 x t
+
+end LambdaCalculus.LocallyNameless.Untyped.Term
+
+end Cslib
diff --git a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/StrongNorm.lean b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/StrongNorm.lean
index 550f63e1e7..c96b214eb4 100644
--- a/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/StrongNorm.lean
+++ b/Cslib/Languages/LambdaCalculus/LocallyNameless/Untyped/StrongNorm.lean
@@ -6,9 +6,8 @@ Authors: David Wegmann
 
 module
 
-public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.FullBeta
 public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.MultiApp
-public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.LcAt
+public import Cslib.Languages.LambdaCalculus.LocallyNameless.Untyped.ParEtaC
 public import Cslib.Foundations.Relation.Confluence
 
 /-! Strong normalization (termination) for full beta-reduction of untyped lambda calculus. -/
@@ -158,6 +157,14 @@ lemma sn_abs_app_multiApp [DecidableEq Var] [HasFresh Var] {Ps} {M N : Term Var}
               refine Relation.TransGen.single (Xi.base (Beta.beta ?_ ?_))
               all_goals grind
 
+theorem sn_eta_step_inv [DecidableEq Var] [HasFresh Var]
+  (t_st_t' : t ⭢ηᶠ t') (sn_t' : SN FullBeta t') : SN FullBeta t :=
+    sn_transfer sn_t' (parEtaC_of_fullEta t_st_t')
+
+theorem sn_eta_steps_inv [DecidableEq Var] [HasFresh Var]
+  (t_st_t' : t ↠ηᶠ t') (sn_t' : SN FullBeta t') : SN FullBeta t := by
+    induction t_st_t' with grind [sn_eta_step_inv]
+
 end LambdaCalculus.LocallyNameless.Untyped.Term
 
 end Cslib