diff --git a/Algorithm/Data/MutableQuotient.lean b/Algorithm/Data/MutableQuotient.lean
index 319832e..b52e77f 100644
--- a/Algorithm/Data/MutableQuotient.lean
+++ b/Algorithm/Data/MutableQuotient.lean
@@ -34,20 +34,22 @@ namespace MutableQuotient
 def mk (m : α → M) (a : α) : MutableQuotient α m := Mutable.mk ⟦a⟧
 
 @[inline]
-def get (x : MutableQuotient α m) (f : α → β) (hf : ∀ a₁ a₂, m a₁ = m a₂ → f a₁ = f a₂) : β :=
+def lift (x : MutableQuotient α m) (f : α → β) (hf : ∀ a₁ a₂, m a₁ = m a₂ → f a₁ = f a₂) : β :=
   (Mutable.get x).lift f fun _ _ h ↦ hf _ _ (by exact h)
 
 @[simp]
-lemma mk_get (m : α → M) (a : α) (f : α → β) (hf : ∀ a₁ a₂, m a₁ = m a₂ → f a₁ = f a₂) :
-    (mk m a).get f hf = f a :=
-  rfl
+lemma mk_lift (m : α → M) (a : α) (f : α → β) (hf : ∀ a₁ a₂, m a₁ = m a₂ → f a₁ = f a₂) :
+    (mk m a).lift f hf = f a := by
+  -- rfl
+  simp [mk, lift]
 
 @[elab_as_elim, induction_eliminator]
 lemma ind {motive : MutableQuotient α m → Prop} (h : ∀ (a : α), motive (mk m a)) (x) : motive x :=
-  Quotient.ind (motive := fun x ↦ motive (Mutable.mk x)) h (Mutable.get x)
+  -- Quotient.ind (motive := fun x ↦ motive (Mutable.mk x)) h (Mutable.get x)
+  Mutable.rec (Quotient.ind h) x
 
 @[inline]
-def getMkEq (x : MutableQuotient α m) (f : ∀ a : α, m a = x.get m (fun _ _ ↦ id) → β)
+def liftOnMkEq (x : MutableQuotient α m) (f : ∀ a : α, m a = x.lift m (fun _ _ ↦ id) → β)
     (hf : ∀ a₁ ha₁ a₂ ha₂, f a₁ ha₁ = f a₂ ha₂) : β :=
   (Mutable.get x).liftOnMkEq (fun a ha ↦ f a (congr_arg (Quotient.lift m _) ha))
     (fun _ _ _ _ ↦ hf _ _ _ _)
@@ -55,24 +57,20 @@ def getMkEq (x : MutableQuotient α m) (f : ∀ a : α, m a = x.get m (fun _ _ 
 @[inline]
 def map (x : MutableQuotient α m) (f : α → α) (hf : ∀ a₁ a₂, m a₁ = m a₂ → m (f a₁) = m (f a₂)) :
     MutableQuotient α m :=
-  get x (fun a ↦ mk m (f a)) (Mutable.ext <| Quotient.sound <| hf · · ·)
-
--- def getModify (x : MutableQuotient α m) (f : α → β) (hf : ∀ a₁ a₂, m a₁ = m a₂ → f a₁ = f a₂)
---     (r : α → α) (hr : ∀ a, m (r a) = m a) : β :=
---   (Mutable.getModify x (Quotient.map' r (fun a₁ a₂ h ↦ (hr a₁).trans h |>.trans (hr a₂).symm))
---     (Quotient.ind fun a ↦ Quotient.sound (hr a))).lift f fun _ _ h ↦ hf _ _ (by exact h)
+  lift x (fun a ↦ mk m (f a)) (Mutable.mk_inj.mpr <| Quotient.sound <| hf · · ·)
 
 @[inline]
-def getModify (x : MutableQuotient α m) (fr : α → β × α)
+def liftModify (x : MutableQuotient α m) (fr : α → β × α)
     (hf : ∀ a₁ a₂, m a₁ = m a₂ → (fr a₁).fst = (fr a₂).fst) (hr : ∀ a, m (fr a).snd = m a) : β :=
-  Mutable.getModify₂ x (Quotient.lift (let ⟨x, y⟩ := fr ·; (x, ⟦y⟧)) (fun a₁ a₂ h ↦
+  Mutable.getModify x (Quotient.lift (let ⟨x, y⟩ := fr ·; (x, ⟦y⟧)) (fun a₁ a₂ h ↦
       Prod.ext (hf a₁ a₂ h) <| Quotient.sound <| (hr a₁).trans <| h.trans (hr a₂).symm))
     (Quotient.ind fun a ↦ Quotient.sound (hr a))
 
 @[simp]
-lemma mk_getModify (m : α → M) (a : α) (fr : α → β × α)
+lemma mk_liftModify (m : α → M) (a : α) (fr : α → β × α)
     (hf : ∀ a₁ a₂, m a₁ = m a₂ → (fr a₁).fst = (fr a₂).fst) (hr : ∀ a, m (fr a).snd = m a) :
-    (mk m a).getModify fr hf hr = (fr a).fst :=
-  rfl
+    (mk m a).liftModify fr hf hr = (fr a).fst := by
+  -- rfl
+  simp [mk, liftModify]
 
 end MutableQuotient
diff --git a/Algorithm/Data/UnionFind.lean b/Algorithm/Data/UnionFind.lean
index 6bac679..68ecaf1 100644
--- a/Algorithm/Data/UnionFind.lean
+++ b/Algorithm/Data/UnionFind.lean
@@ -457,7 +457,7 @@ instance [OfFn P ι ι id] [OfFn S ι ℕ (fun _ ↦ 1)] : Inhabited (UnionFind
 
 @[inline]
 def find (self : UnionFind ι P S) (i : ι) : ι :=
-  MutableQuotient.getModify self (fun x ↦ x.find i) (by simp (config := { contextual := true }))
+  MutableQuotient.liftModify self (fun x ↦ x.find i) (by simp (config := { contextual := true }))
     (by simp)
 
 def IsRoot (self : UnionFind ι P S) (i : ι) : Prop := self.find i = i
@@ -477,7 +477,7 @@ def union (self : UnionFind ι P S) (i j : ι) : UnionFind ι P S :=
 
 @[inline]
 def size (self : UnionFind ι P S) (i : ι) (hi : self.IsRoot i) : ℕ :=
-  MutableQuotient.getMkEq self
+  MutableQuotient.liftOnMkEq self
     (fun x hx ↦ x.size i (by
       induction self using MutableQuotient.ind
       rw [UnionFindImpl.UnionFindWF.isRoot_iff_root, hx, ← hi]
diff --git a/Mutable/Mutable.lean b/Mutable/Mutable.lean
index 7c81423..52468cc 100644
--- a/Mutable/Mutable.lean
+++ b/Mutable/Mutable.lean
@@ -17,67 +17,71 @@ structure Mutable (α : Type u) : Type u where
   mk ::
   get : α
 
-attribute [extern "lean_mk_Mutable"] Mutable.mk
-attribute [extern "lean_Mutable_get"] Mutable.get
-
-variable {α : Type u}
-
-@[extern "lean_Mutable_get_with"]
-protected def Mutable.getWith (x : @& Mutable α) (f : α → α) : α :=
-  x.get
+attribute [extern "lean_st_mk_ref"] Mutable.mk
+attribute [extern "lean_st_ref_get"] Mutable.get
 -/
 
-structure Mutable (α : Type u) : Type u where
-  private __mk__ ::
-  private __get__ : α
+opaque MutableAux (α : Type u) : Subtype (· = α) := ⟨α, rfl⟩
+
+def Mutable (α : Type u) : Type u := (MutableAux α).val
 
 namespace Mutable
 variable {α : Type u} {β : Type v}
 
-@[extern "lean_mk_Mutable"]
-def mk (a : α) : Mutable α := __mk__ a
+@[extern "lean_st_mk_ref"]
+def mk (a : α) : Mutable α := (MutableAux α).2.mpr a
+
+@[extern "lean_st_ref_get", never_extract]
+def get (x : @& Mutable α) : α := (MutableAux α).2.mp x
 
-@[extern "lean_Mutable_get"]
-def get (a : @& Mutable α) : α := __get__ a
+set_option linter.unusedVariables false in
+@[extern "lean_Mutable_set", never_extract]
+unsafe def set (x : @& Mutable α) (a : α) (b : @& β) : β := b
+
+@[simp] theorem mk_get (x : Mutable α) : mk x.get = x := by simp [mk, Mutable.get]
+@[simp] theorem get_mk (x : α) : (mk x).get = x := by simp [mk, Mutable.get]
+
+def rec {motive : Mutable α → Sort _} (h : ∀ a, motive (mk a)) (x : Mutable α) : motive x :=
+  mk_get x ▸ h _
 
-theorem ext {x y : Mutable α} (get : x.get = y.get) : x = y :=
-  match x, y, get with | ⟨_⟩, ⟨_⟩, rfl => rfl
+theorem ext {x y : Mutable α} (get : x.get = y.get) : x = y := by
+  simpa [Mutable.get] using congrArg (MutableAux α).2.mpr get
 
 theorem ext_iff {x y : Mutable α} : x = y ↔ x.get = y.get :=
   ⟨congrArg get, ext⟩
 
 @[simp]
-theorem mk_eq_mk {x y : α} : mk x = mk y ↔ x = y :=
-  ext_iff
+theorem mk_inj {x y : α} : mk x = mk y ↔ x = y :=
+  ext_iff.trans (by simp)
 
-set_option linter.unusedVariables false in
-@[extern "lean_Mutable_modify"]
-unsafe def getModifyUnsafe (x : @& Mutable α) (f : α → α) : α :=
-  x.get
+unsafe def modifyUnsafe (x : Mutable α) (f : α → α) : α :=
+  let a := f x.get; x.set a a
 
 set_option linter.unusedVariables false in
-unsafe abbrev getModifyImpl (x : Mutable α)
+unsafe abbrev modifyImpl (x : Mutable α)
     (f : α → α) (hf : ∀ a, f a = a) : α :=
-  Mutable.getModifyUnsafe x f
+  Mutable.modifyUnsafe x f
 
-@[implemented_by Mutable.getModifyImpl]
-def getModify (x : Mutable α)
+@[implemented_by Mutable.modifyImpl]
+def modify (x : Mutable α)
     (f : α → α) (hf : ∀ a, f a = a) : α :=
   f x.get
 
-set_option linter.unusedVariables false in
-@[extern "lean_Mutable_modify2"]
-unsafe def getModify₂Unsafe (x : @& Mutable α) (f : α → β × α) : β :=
-  (f x.get).fst
+unsafe def getModifyUnsafe (x : Mutable α) (f : α → β × α) : β :=
+  let (b, a) := f x.get; x.set a b
 
 set_option linter.unusedVariables false in
-unsafe abbrev getModify₂Impl (x : Mutable α)
+unsafe abbrev getModifyImpl (x : Mutable α)
     (f : α → β × α) (hgf : ∀ a, (f a).snd = a) : β :=
-  Mutable.getModify₂Unsafe x f
+  Mutable.getModifyUnsafe x f
 
-@[implemented_by Mutable.getModify₂Impl]
-def getModify₂ (x : Mutable α)
-    (f : α → β × α) (hgf : ∀ a, (f a).snd = a) : β :=
+@[implemented_by Mutable.getModifyImpl]
+def getModify (x : Mutable α) (f : α → β × α) (hgf : ∀ a, (f a).snd = a) : β :=
   (f x.get).fst
 
+@[simp]
+theorem getModify_mk {a : α} {f : α → β × α} {hgf : ∀ a, (f a).snd = a} :
+    (mk a).getModify f hgf = (f a).fst := by
+  simp [getModify]
+
 end Mutable
diff --git a/cpp/ffi.cpp b/cpp/ffi.cpp
index df79f2a..03ae853 100644
--- a/cpp/ffi.cpp
+++ b/cpp/ffi.cpp
@@ -1,68 +1,6 @@
 #include <lean/lean.h>
 
-struct Mutable {
-    std::atomic<lean_object *> m_value;
-    Mutable(lean_object * v) : m_value(v) { }
-};
-
-static void Mutable_finalize(void * o) {
-    lean_dec(static_cast<Mutable *>(o)->m_value);
-    delete static_cast<Mutable *>(o);
-}
-
-static void Mutable_foreach(void * o, b_lean_obj_arg f) {
-    lean_inc(f);
-    lean_apply_1(f, static_cast<Mutable *>(o)->m_value);
-}
-
-static lean_external_class * g_Mutable_class = nullptr;
-
-static inline lean_object * Mutable_to_lean(Mutable * x) {
-    if (g_Mutable_class == nullptr) {
-        g_Mutable_class = lean_register_external_class(Mutable_finalize, Mutable_foreach);
-    }
-    return lean_alloc_external(g_Mutable_class, x);
-}
-
-static inline Mutable * lean_to_Mutable(b_lean_obj_arg o) {
-    return static_cast<Mutable *>(lean_get_external_data(o));
-}
-
-extern "C" LEAN_EXPORT lean_obj_res lean_mk_Mutable(b_lean_obj_arg o) {
-    return Mutable_to_lean(new Mutable(o));
-}
-
-extern "C" LEAN_EXPORT b_lean_obj_res lean_Mutable_get(b_lean_obj_arg x) {
-    lean_inc(lean_to_Mutable(x)->m_value);
-    return lean_to_Mutable(x)->m_value;
-}
-
-extern "C" LEAN_EXPORT b_lean_obj_res lean_Mutable_modify(b_lean_obj_arg x, lean_obj_arg f) {
-    lean_object * c = lean_to_Mutable(x)->m_value.exchange(nullptr);
-    while (c == nullptr) {
-        // std::this_thread::yield();
-        c = lean_to_Mutable(x)->m_value.exchange(nullptr);
-    }
-    lean_object * r = lean_apply_1(f, c);
-    // lean_assert(r != nullptr); /* Closure must return a valid lean object */
-    // lean_assert(lean_to_Mutable(x)->m_value == nullptr);
-    lean_inc(r);
-    lean_to_Mutable(x)->m_value = r;
-    return r;
-}
-
-extern "C" LEAN_EXPORT b_lean_obj_res lean_Mutable_modify2(b_lean_obj_arg x, lean_obj_arg f) {
-    lean_object * c = lean_to_Mutable(x)->m_value.exchange(nullptr);
-    while (c == nullptr) {
-        // std::this_thread::yield();
-        c = lean_to_Mutable(x)->m_value.exchange(nullptr);
-    }
-    lean_object * p = lean_apply_1(f, c);
-    // lean_assert(p != nullptr); /* Closure must return a valid lean object */
-    // lean_assert(lean_to_Mutable(x)->m_value == nullptr);
-    lean_object * r = lean_ctor_get(p, 0); lean_inc(r);
-    lean_object * n = lean_ctor_get(p, 1); lean_inc(n);
-    lean_dec_ref(p);
-    lean_to_Mutable(x)->m_value = n;
-    return r;
+extern "C" LEAN_EXPORT b_lean_obj_res lean_Mutable_set(b_lean_obj_arg ref, lean_obj_arg a, b_lean_obj_arg b) {
+    lean_st_ref_set(ref, a);
+    return b;
 }
diff --git a/scripts/nolints.json b/scripts/nolints.json
index 22297a4..a289aba 100644
--- a/scripts/nolints.json
+++ b/scripts/nolints.json
@@ -113,9 +113,9 @@
  ["docBlame", "MinHeap.headD"],
  ["docBlame", "MinHeap.tail"],
  ["docBlame", "MultiBag.ReadOnly"],
- ["docBlame", "MutableQuotient.get"],
- ["docBlame", "MutableQuotient.getMkEq"],
- ["docBlame", "MutableQuotient.getModify"],
+ ["docBlame", "MutableQuotient.lift"],
+ ["docBlame", "MutableQuotient.liftModify"],
+ ["docBlame", "MutableQuotient.liftOnMkEq"],
  ["docBlame", "MutableQuotient.map"],
  ["docBlame", "MutableQuotient.mk"],
  ["docBlame", "OfFn.ofFn"],
diff --git a/test/Mutable/thunk1.lean b/test/Mutable/thunk1.lean
index cba7945..66d91b8 100644
--- a/test/Mutable/thunk1.lean
+++ b/test/Mutable/thunk1.lean
@@ -12,7 +12,7 @@ protected def Thunk'.pure (a : α) : Thunk' α :=
   .mk fun _ ↦ a
 
 protected def Thunk'.get (x : Thunk' α) : α :=
-  Mutable.getModify₂ x (fun f ↦ let a := f (); ⟨a, fun _ ↦ a⟩) (fun _ ↦ rfl)
+  Mutable.getModify x (fun f ↦ let a := f (); ⟨a, fun _ ↦ a⟩) (fun _ ↦ rfl)
 
 @[inline] protected def Thunk'.map (f : α → β) (x : Thunk' α) : Thunk' β :=
   .mk fun _ => f x.get
@@ -23,46 +23,43 @@ protected def Thunk'.get (x : Thunk' α) : α :=
 /-! lean4/tests/lean/thunk.lean -/
 
 /-- info: 1 -/
-#guard_msgs in #eval
-  (Thunk'.pure 1).get
+#guard_msgs in #eval (Thunk'.pure 1).get
 /-- info: 2 -/
 #guard_msgs in #eval (Thunk'.mk fun _ => 2).get
 /--
 info: 3
 4
-5
+---
+info: 5
 -/
 #guard_msgs in #eval
   let t1 := Thunk'.mk fun _ => dbg_trace 4; 5
-  -- let t2 := Thunk'.mk fun _ => dbg_trace 3; 0
-  -- let v2 := t2.get
-  let v2 := dbg_trace 3; 0
-
+  let t2 := Thunk'.mk fun _ => dbg_trace 3; 0
+  let v2 := t2.get
   let v1 := t1.get
   v1 + v2
 /--
 info: 6
 7
-8
+---
+info: 8
 -/
 #guard_msgs in #eval
   let t1 := Thunk'.pure 8 |>.map fun n => dbg_trace 7; n
-  -- let t2 := Thunk'.mk fun _ => dbg_trace 6; 0
-  -- let v2 := t2.get
-  let v2 := dbg_trace 6; 0
-
+  let t2 := Thunk'.mk fun _ => dbg_trace 6; 0
+  let v2 := t2.get
   let v1 := t1.get
   v1 + v2
+
 /--
 info: 9
 10
-11
+---
+info: 11
 -/
 #guard_msgs in #eval
   let t1 := Thunk'.pure 11 |>.bind fun n => dbg_trace 10; Thunk'.pure n
-  -- let t2 := Thunk'.mk fun _ => dbg_trace 9; 0
-  -- let v2 := t2.get
-  let v2 := dbg_trace 9; 0
-
+  let t2 := Thunk'.mk fun _ => dbg_trace 9; 0
+  let v2 := t2.get
   let v1 := t1.get
   v1 + v2
diff --git a/test/Mutable/thunk2.lean b/test/Mutable/thunk2.lean
index b80da62..1be122a 100644
--- a/test/Mutable/thunk2.lean
+++ b/test/Mutable/thunk2.lean
@@ -12,7 +12,7 @@ protected def Thunk'.pure (a : α) : Thunk' α :=
   .mk fun _ ↦ a
 
 protected def Thunk'.get (x : Thunk' α) : α :=
-  Mutable.getModify₂ x (fun f ↦ let a := f (); ⟨a, fun _ ↦ a⟩) (fun _ ↦ rfl)
+  Mutable.getModify x (fun f ↦ let a := f (); ⟨a, fun _ ↦ a⟩) (fun _ ↦ rfl)
 
 /-! lean4/tests/compiler/thunk.lean -/
 
@@ -27,4 +27,4 @@ def main : IO Unit :=
   IO.println (toString (test (compute 1) 100000))
 
 /-- info: 10000000000 -/
-#guard_msgs in #eval main -- TODO: 超时中断
+#guard_msgs in #eval main
diff --git a/test/Mutable/thunk3.lean b/test/Mutable/thunk3.lean
index a9ab360..3f47cf4 100644
--- a/test/Mutable/thunk3.lean
+++ b/test/Mutable/thunk3.lean
@@ -12,7 +12,7 @@ protected def Thunk'.pure (a : α) : Thunk' α :=
   .mk fun _ ↦ a
 
 protected def Thunk'.get (x : Thunk' α) : α :=
-  Mutable.getModify₂ x (fun f ↦ let a := f (); ⟨a, fun _ ↦ a⟩) (fun _ ↦ rfl)
+  Mutable.getModify x (fun f ↦ let a := f (); ⟨a, fun _ ↦ a⟩) (fun _ ↦ rfl)
 
 @[inline] protected def Thunk'.map (f : α → β) (x : Thunk' α) : Thunk' β :=
   .mk fun _ => f x.get
@@ -20,9 +20,6 @@ protected def Thunk'.get (x : Thunk' α) : α :=
 @[inline] protected def Thunk'.bind (x : Thunk' α) (f : α → Thunk' β) : Thunk' β :=
   .mk fun _ => (f x.get).get
 
-def List.sum [OfNat α 0] [Add α] : List α → α :=
-  foldl (· + ·) 0
-
 #eval show IO Unit from do
   let _ : OfNat (Thunk' Nat) 0 := ⟨.pure 0⟩
   let _ : Inhabited (Thunk' Nat) := ⟨.pure 0⟩