From 213cb18da7a103aac1084e486efde2fdb412c361 Mon Sep 17 00:00:00 2001
From: Jireh Loreaux <loreaujy@gmail.com>
Date: Tue, 8 Oct 2024 09:32:33 -0500
Subject: [PATCH 1/2] refactor: migrate to dependent induction lemmas

---
 Mathlib/Algebra/Group/Submonoid/Basic.lean    | 51 ++++++++++---------
 Mathlib/Algebra/Group/Subsemigroup/Basic.lean | 49 +++++++++---------
 2 files changed, 52 insertions(+), 48 deletions(-)

diff --git a/Mathlib/Algebra/Group/Submonoid/Basic.lean b/Mathlib/Algebra/Group/Submonoid/Basic.lean
index ddb7f8577ed46..f75d0f5d2e4ff 100644
--- a/Mathlib/Algebra/Group/Submonoid/Basic.lean
+++ b/Mathlib/Algebra/Group/Submonoid/Basic.lean
@@ -372,32 +372,36 @@ is preserved under multiplication, then `p` holds for all elements of the closur
       "An induction principle for additive closure membership. If `p` holds for `0` and all
       elements of `s`, and is preserved under addition, then `p` holds for all elements of the
       additive closure of `s`."]
-theorem closure_induction {p : M → Prop} {x} (h : x ∈ closure s) (mem : ∀ x ∈ s, p x) (one : p 1)
-    (mul : ∀ x y, p x → p y → p (x * y)) : p x :=
-  (@closure_le _ _ _ ⟨⟨p, mul _ _⟩, one⟩).2 mem h
-
-/-- A dependent version of `Submonoid.closure_induction`. -/
-@[to_additive (attr := elab_as_elim) "A dependent version of `AddSubmonoid.closure_induction`. "]
-theorem closure_induction' (s : Set M) {p : ∀ x, x ∈ closure s → Prop}
+theorem closure_induction {s : Set M} {p : ∀ x, x ∈ closure s → Prop}
     (mem : ∀ (x) (h : x ∈ s), p x (subset_closure h)) (one : p 1 (one_mem _))
     (mul : ∀ x hx y hy, p x hx → p y hy → p (x * y) (mul_mem hx hy)) {x} (hx : x ∈ closure s) :
-    p x hx := by
-  refine Exists.elim ?_ fun (hx : x ∈ closure s) (hc : p x hx) => hc
-  exact
-    closure_induction hx (fun x hx => ⟨_, mem x hx⟩) ⟨_, one⟩ fun x y ⟨hx', hx⟩ ⟨hy', hy⟩ =>
-      ⟨_, mul _ _ _ _ hx hy⟩
+    p x hx :=
+  let S : Submonoid M :=
+    { carrier := { x | ∃ hx, p x hx }
+      one_mem' := ⟨_, one⟩
+      mul_mem' := fun ⟨hx, hpx⟩ ⟨hy, hpy⟩ ↦ ⟨_, mul _ hx _ hy hpx hpy⟩ }
+  closure_le (S := S) |>.mpr (fun y hy ↦ ⟨subset_closure hy, mem y hy⟩) hx |>.elim fun _ ↦ id
 
 /-- An induction principle for closure membership for predicates with two arguments. -/
 @[to_additive (attr := elab_as_elim)
       "An induction principle for additive closure membership for predicates with two arguments."]
-theorem closure_induction₂ {p : M → M → Prop} {x} {y : M} (hx : x ∈ closure s) (hy : y ∈ closure s)
-    (Hs : ∀ x ∈ s, ∀ y ∈ s, p x y) (H1_left : ∀ x, p 1 x) (H1_right : ∀ x, p x 1)
-    (Hmul_left : ∀ x y z, p x z → p y z → p (x * y) z)
-    (Hmul_right : ∀ x y z, p z x → p z y → p z (x * y)) : p x y :=
-  closure_induction hx
-    (fun x xs =>
-      closure_induction hy (Hs x xs) (H1_right x) fun z _ h₁ h₂ => Hmul_right z _ _ h₁ h₂)
-    (H1_left y) fun _ _ h₁ h₂ => Hmul_left _ _ _ h₁ h₂
+theorem closure_induction₂ {p : ∀ x ∈ closure s, ∀ y, y ∈ closure s → Prop}
+    (mem : ∀ (x) (hx : x ∈ s) (y) (hy : y ∈ s), p x (subset_closure hx) y (subset_closure hy))
+    (one_left : ∀ x hx, p 1 (one_mem _) x hx) (one_right : ∀ x hx, p x hx 1 (one_mem _))
+    (mul_left : ∀ x hx y hy z hz, p x hx z hz → p y hy z hz → p (x * y) (mul_mem hx hy) z hz)
+    (mul_right : ∀ x hx y hy z hz, p z hz x hx → p z hz y hy → p z hz (x * y) (mul_mem hx hy))
+    {x y : M} (hx : x ∈ closure s) (hy : y ∈ closure s) : p x hx y hy := by
+  refine closure_induction (closure_induction mem (fun z hz ↦ one_left z (subset_closure hz)) ?_ hx)
+    (one_right x hx) (mul_right · · · · _ hx · ·) hy
+  exact fun _ _ _ _ h₁ h₂ z hz ↦ mul_left _ _ _ _ _ (subset_closure hz) (h₁ _ hz) (h₂ _ hz)
+-- which version do we prefer?
+  --induction hy using closure_induction with
+    --| mem z hz => induction hx using closure_induction with
+      --| mem _ h => exact mem _ h _ hz
+      --| one => exact one_left _ (subset_closure hz)
+      --| mul _ _ _ _ h₁ h₂ => exact mul_left _ _ _ _ _ (subset_closure hz) h₁ h₂
+    --| one => exact one_right x hx
+    --| mul _ _ _ _ h₁ h₂ => exact mul_right _ _ _ _ _ hx h₁ h₂
 
 /-- If `s` is a dense set in a monoid `M`, `Submonoid.closure s = ⊤`, then in order to prove that
 some predicate `p` holds for all `x : M` it suffices to verify `p x` for `x ∈ s`, verify `p 1`,
@@ -408,16 +412,15 @@ and verify that `p x` and `p y` imply `p (x * y)`. -/
       `x ∈ s`, verify `p 0`, and verify that `p x` and `p y` imply `p (x + y)`."]
 theorem dense_induction {p : M → Prop} (x : M) {s : Set M} (hs : closure s = ⊤)
     (mem : ∀ x ∈ s, p x)
-    (one : p 1) (mul : ∀ x y, p x → p y → p (x * y)) : p x := by
-  have : ∀ x ∈ closure s, p x := fun x hx => closure_induction hx mem one mul
-  simpa [hs] using this x
+    (one : p 1) (mul : ∀ x y, p x → p y → p (x * y)) : p x :=
+  closure_induction (p := fun x _ ↦ p x) mem one (fun x _ y _ ↦ mul x y) (hs.symm ▸ mem_top x)
 
 /-- The `Submonoid.closure` of a set is the union of `{1}` and its `Subsemigroup.closure`. -/
 lemma closure_eq_one_union (s : Set M) :
     closure s = {(1 : M)} ∪ (Subsemigroup.closure s : Set M) := by
   apply le_antisymm
   · intro x hx
-    induction hx using closure_induction' with
+    induction hx using closure_induction with
     | mem x hx => exact Or.inr <| Subsemigroup.subset_closure hx
     | one => exact Or.inl <| by simp
     | mul x hx y hy hx hy =>
diff --git a/Mathlib/Algebra/Group/Subsemigroup/Basic.lean b/Mathlib/Algebra/Group/Subsemigroup/Basic.lean
index 88cda9b9d5583..04df3e5724acc 100644
--- a/Mathlib/Algebra/Group/Subsemigroup/Basic.lean
+++ b/Mathlib/Algebra/Group/Subsemigroup/Basic.lean
@@ -294,30 +294,31 @@ is preserved under multiplication, then `p` holds for all elements of the closur
 @[to_additive (attr := elab_as_elim) "An induction principle for additive closure membership. If `p`
   holds for all elements of `s`, and is preserved under addition, then `p` holds for all
   elements of the additive closure of `s`."]
-theorem closure_induction {p : M → Prop} {x} (h : x ∈ closure s) (mem : ∀ x ∈ s, p x)
-    (mul : ∀ x y, p x → p y → p (x * y)) : p x :=
-  (@closure_le _ _ _ ⟨p, mul _ _⟩).2 mem h
-
-/-- A dependent version of `Subsemigroup.closure_induction`. -/
-@[to_additive (attr := elab_as_elim) "A dependent version of `AddSubsemigroup.closure_induction`. "]
-theorem closure_induction' (s : Set M) {p : ∀ x, x ∈ closure s → Prop}
+theorem closure_induction {p : ∀ x, x ∈ closure s → Prop}
     (mem : ∀ (x) (h : x ∈ s), p x (subset_closure h))
     (mul : ∀ x hx y hy, p x hx → p y hy → p (x * y) (mul_mem hx hy)) {x} (hx : x ∈ closure s) :
-    p x hx := by
-  refine Exists.elim ?_ fun (hx : x ∈ closure s) (hc : p x hx) => hc
-  exact
-    closure_induction hx (fun x hx => ⟨_, mem x hx⟩) fun x y ⟨hx', hx⟩ ⟨hy', hy⟩ =>
-      ⟨_, mul _ _ _ _ hx hy⟩
+    p x hx :=
+  let S : Subsemigroup M :=
+    { carrier := { x | ∃ hx, p x hx }
+      mul_mem' := fun ⟨hx, hpx⟩ ⟨hy, hpy⟩ ↦ ⟨mul_mem hx hy, mul _ hx _ hy hpx hpy⟩ }
+  closure_le (S := S) |>.mpr (fun y hy ↦ ⟨subset_closure hy, mem y hy⟩) hx |>.elim fun _ ↦ id
 
 /-- An induction principle for closure membership for predicates with two arguments. -/
 @[to_additive (attr := elab_as_elim) "An induction principle for additive closure membership for
   predicates with two arguments."]
-theorem closure_induction₂ {p : M → M → Prop} {x} {y : M} (hx : x ∈ closure s) (hy : y ∈ closure s)
-    (Hs : ∀ x ∈ s, ∀ y ∈ s, p x y) (Hmul_left : ∀ x y z, p x z → p y z → p (x * y) z)
-    (Hmul_right : ∀ x y z, p z x → p z y → p z (x * y)) : p x y :=
-  closure_induction hx
-    (fun x xs => closure_induction hy (Hs x xs) fun z _ h₁ h₂ => Hmul_right z _ _ h₁ h₂)
-    fun _ _ h₁ h₂ => Hmul_left _ _ _ h₁ h₂
+theorem closure_induction₂ {p : ∀ x ∈ closure s, ∀ y, y ∈ closure s → Prop}
+    (mem : ∀ (x) (hx : x ∈ s) (y) (hy : y ∈ s), p x (subset_closure hx) y (subset_closure hy))
+    (mul_left : ∀ x hx y hy z hz , p x hx z hz → p y hy z hz → p (x * y) (mul_mem hx hy) z hz)
+    (mul_right : ∀ x hx y hy z hz , p z hz x hx → p z hz y hy → p z hz (x * y) (mul_mem hx hy))
+    {x y : M} (hx : x ∈ closure s) (hy : y ∈ closure s) : p x hx y hy := by
+  refine closure_induction (closure_induction mem ?_ hx) (mul_right · · · · _ hx · ·) hy
+  exact fun _ _ _ _ h₁ h₂ z hz ↦ mul_left _ _ _ _ _ (subset_closure hz) (h₁ _ hz) (h₂ _ hz)
+-- which version do we prefer?
+--induction hx using closure_induction with
+  --| mem z hz => induction hy using closure_induction with
+    --| mem _ h => exact mem _ hz _ h
+    --| mul _ _ _ _ h₁ h₂ => exact mul_right _ _ _ _ _ (subset_closure hz) h₁ h₂
+  --| mul _ _ _ _ h₁ h₂ => exact mul_left _ _ _ _ _ hy h₁ h₂
 
 /-- If `s` is a dense set in a magma `M`, `Subsemigroup.closure s = ⊤`, then in order to prove that
 some predicate `p` holds for all `x : M` it suffices to verify `p x` for `x ∈ s`,
@@ -326,11 +327,11 @@ and verify that `p x` and `p y` imply `p (x * y)`. -/
   `AddSubsemigroup.closure s = ⊤`, then in order to prove that some predicate `p` holds
   for all `x : M` it suffices to verify `p x` for `x ∈ s`, and verify that `p x` and `p y` imply
   `p (x + y)`."]
-theorem dense_induction {p : M → Prop} (x : M) {s : Set M} (hs : closure s = ⊤)
+theorem dense_induction {p : M → Prop} {s : Set M} (hs : closure s = ⊤)
     (mem : ∀ x ∈ s, p x)
-    (mul : ∀ x y, p x → p y → p (x * y)) : p x := by
-  have : ∀ x ∈ closure s, p x := fun x hx => closure_induction hx mem mul
-  simpa [hs] using this x
+    (mul : ∀ x y, p x → p y → p (x * y))
+    (x : M) : p x :=
+  closure_induction (p := fun x _ ↦ p x) mem (fun x _ y _ ↦ mul x y) (hs.symm ▸ mem_top x)
 
 variable (M)
 
@@ -426,8 +427,8 @@ def ofDense {M N} [Semigroup M] [Semigroup N] {s : Set M} (f : M → N) (hs : cl
     M →ₙ* N where
   toFun := f
   map_mul' x y :=
-    dense_induction y hs (fun y hy x => hmul x y hy)
-      (fun y₁ y₂ h₁ h₂ x => by simp only [← mul_assoc, h₁, h₂]) x
+    dense_induction hs (fun y hy x => hmul x y hy)
+      (fun y₁ y₂ h₁ h₂ x => by simp only [← mul_assoc, h₁, h₂]) y x
 
 /-- Let `s` be a subset of an additive semigroup `M` such that the closure of `s` is the whole
 semigroup.  Then `AddHom.ofDense` defines an additive homomorphism from `M` asking for a proof

From 309fd33d76d9865279ab68faa2fb334ad5700f6d Mon Sep 17 00:00:00 2001
From: Jireh Loreaux <loreaujy@gmail.com>
Date: Tue, 8 Oct 2024 09:49:02 -0500
Subject: [PATCH 2/2] =?UTF-8?q?fix=20`closure=5Finduction=E2=82=82`?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 Mathlib/Algebra/Group/Submonoid/Basic.lean    | 12 ++++++------
 Mathlib/Algebra/Group/Subsemigroup/Basic.lean | 10 +++++-----
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/Mathlib/Algebra/Group/Submonoid/Basic.lean b/Mathlib/Algebra/Group/Submonoid/Basic.lean
index f75d0f5d2e4ff..063566547110c 100644
--- a/Mathlib/Algebra/Group/Submonoid/Basic.lean
+++ b/Mathlib/Algebra/Group/Submonoid/Basic.lean
@@ -385,12 +385,12 @@ theorem closure_induction {s : Set M} {p : ∀ x, x ∈ closure s → Prop}
 /-- An induction principle for closure membership for predicates with two arguments. -/
 @[to_additive (attr := elab_as_elim)
       "An induction principle for additive closure membership for predicates with two arguments."]
-theorem closure_induction₂ {p : ∀ x ∈ closure s, ∀ y, y ∈ closure s → Prop}
-    (mem : ∀ (x) (hx : x ∈ s) (y) (hy : y ∈ s), p x (subset_closure hx) y (subset_closure hy))
-    (one_left : ∀ x hx, p 1 (one_mem _) x hx) (one_right : ∀ x hx, p x hx 1 (one_mem _))
-    (mul_left : ∀ x hx y hy z hz, p x hx z hz → p y hy z hz → p (x * y) (mul_mem hx hy) z hz)
-    (mul_right : ∀ x hx y hy z hz, p z hz x hx → p z hz y hy → p z hz (x * y) (mul_mem hx hy))
-    {x y : M} (hx : x ∈ closure s) (hy : y ∈ closure s) : p x hx y hy := by
+theorem closure_induction₂ {p : ∀ x y, x ∈ closure s → y ∈ closure s → Prop}
+    (mem : ∀ (x) (hx : x ∈ s) (y) (hy : y ∈ s), p x y (subset_closure hx) (subset_closure hy))
+    (one_left : ∀ x hx, p 1 x (one_mem _) hx) (one_right : ∀ x hx, p x 1 hx (one_mem _))
+    (mul_left : ∀ x hx y hy z hz, p x z hx hz → p y z hy hz → p (x * y) z (mul_mem hx hy) hz)
+    (mul_right : ∀ x hx y hy z hz, p z x hz hx → p z y hz hy → p z (x * y) hz (mul_mem hx hy))
+    {x y : M} (hx : x ∈ closure s) (hy : y ∈ closure s) : p x y hx hy := by
   refine closure_induction (closure_induction mem (fun z hz ↦ one_left z (subset_closure hz)) ?_ hx)
     (one_right x hx) (mul_right · · · · _ hx · ·) hy
   exact fun _ _ _ _ h₁ h₂ z hz ↦ mul_left _ _ _ _ _ (subset_closure hz) (h₁ _ hz) (h₂ _ hz)
diff --git a/Mathlib/Algebra/Group/Subsemigroup/Basic.lean b/Mathlib/Algebra/Group/Subsemigroup/Basic.lean
index 04df3e5724acc..b5db17997f3ed 100644
--- a/Mathlib/Algebra/Group/Subsemigroup/Basic.lean
+++ b/Mathlib/Algebra/Group/Subsemigroup/Basic.lean
@@ -306,11 +306,11 @@ theorem closure_induction {p : ∀ x, x ∈ closure s → Prop}
 /-- An induction principle for closure membership for predicates with two arguments. -/
 @[to_additive (attr := elab_as_elim) "An induction principle for additive closure membership for
   predicates with two arguments."]
-theorem closure_induction₂ {p : ∀ x ∈ closure s, ∀ y, y ∈ closure s → Prop}
-    (mem : ∀ (x) (hx : x ∈ s) (y) (hy : y ∈ s), p x (subset_closure hx) y (subset_closure hy))
-    (mul_left : ∀ x hx y hy z hz , p x hx z hz → p y hy z hz → p (x * y) (mul_mem hx hy) z hz)
-    (mul_right : ∀ x hx y hy z hz , p z hz x hx → p z hz y hy → p z hz (x * y) (mul_mem hx hy))
-    {x y : M} (hx : x ∈ closure s) (hy : y ∈ closure s) : p x hx y hy := by
+theorem closure_induction₂ {p : ∀ x y, x ∈ closure s → y ∈ closure s → Prop}
+    (mem : ∀ (x) (hx : x ∈ s) (y) (hy : y ∈ s), p x y (subset_closure hx) (subset_closure hy))
+    (mul_left : ∀ x hx y hy z hz , p x z hx hz → p y z hy hz → p (x * y) z (mul_mem hx hy) hz)
+    (mul_right : ∀ x hx y hy z hz , p z x hz hx → p z y hz hy → p z (x * y) hz (mul_mem hx hy))
+    {x y : M} (hx : x ∈ closure s) (hy : y ∈ closure s) : p x y hx hy := by
   refine closure_induction (closure_induction mem ?_ hx) (mul_right · · · · _ hx · ·) hy
   exact fun _ _ _ _ h₁ h₂ z hz ↦ mul_left _ _ _ _ _ (subset_closure hz) (h₁ _ hz) (h₂ _ hz)
 -- which version do we prefer?