-- Mathlib/Algebra/Group/UniqueProds/Basic.lean
theorem of_same {G} [Semigroup G] [IsCancelMul G]
    (h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2) :
    UniqueProds G where
  uniqueMul_of_nonempty {A B} hA hB

Proof state:
case intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
g1 : G
h1 : g1 ∈ B * A
g2 : G
h2 : g2 ∈ B * A
hu : UniqueMul (B * A) (B * A) g1 g2
⊢ ∃ a0 ∈ A, ∃ b0 ∈ B, UniqueMul A B a0 b0

Tactic:
obtain ⟨b1, hb1, a1, ha1, rfl⟩ := mem_mul.mp h1

Result:
case intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
g2 : G
h2 : g2 ∈ B * A
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) g2
⊢ ∃ a0 ∈ A, ∃ b0 ∈ B, UniqueMul A B a0 b0

Proof state:
case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
a b : G
ha : a ∈ A
hb : b ∈ B
he : a * b = a1 * b2
hu : b1 * a = b1 * a1 ∧ b * a2 = b2 * a2
⊢ a = a1 ∧ b = b2

Tactic:
exact ⟨mul_left_cancel hu.1, mul_right_cancel hu.2⟩

Result:
no goals

Proof state:
case intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
g2 : G
h2 : g2 ∈ B * A
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) g2
⊢ ∃ a0 ∈ A, ∃ b0 ∈ B, UniqueMul A B a0 b0

Tactic:
obtain ⟨b2, hb2, a2, ha2, rfl⟩ := mem_mul.mp h2

Result:
case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) (b2 * a2)
⊢ ∃ a0 ∈ A, ∃ b0 ∈ B, UniqueMul A B a0 b0

Proof state:
case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) (b2 * a2)
a b : G
ha : a ∈ A
hb : b ∈ B
he : a * b = a1 * b2
⊢ b1 * a * (b * a2) = b1 * a1 * (b2 * a2)

Tactic:
rw [mul_assoc b1, ← mul_assoc a, he, mul_assoc a1, ← mul_assoc b1]

Result:
no goals

Proof state:
case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) (b2 * a2)
a b : G
ha : a ∈ A
hb : b ∈ B
he : a * b = a1 * b2
⊢ a = a1 ∧ b = b2

Tactic:
specialize hu (mul_mem_mul hb1 ha) (mul_mem_mul hb ha2) _

Result:
case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) (b2 * a2)
a b : G
ha : a ∈ A
hb : b ∈ B
he : a * b = a1 * b2
⊢ b1 * a * (b * a2) = b1 * a1 * (b2 * a2)

case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
a b : G
ha : a ∈ A
hb : b ∈ B
he : a * b = a1 * b2
hu : b1 * a = b1 * a1 ∧ b * a2 = b2 * a2
⊢ a = a1 ∧ b = b2

Proof state:
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
⊢ ∃ a0 ∈ A, ∃ b0 ∈ B, UniqueMul A B a0 b0

Tactic:
obtain ⟨g1, h1, g2, h2, hu⟩ := h (hB.mul hA)

Result:
case intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
g1 : G
h1 : g1 ∈ B * A
g2 : G
h2 : g2 ∈ B * A
hu : UniqueMul (B * A) (B * A) g1 g2
⊢ ∃ a0 ∈ A, ∃ b0 ∈ B, UniqueMul A B a0 b0

Proof state:
case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) (b2 * a2)
⊢ ∃ a0 ∈ A, ∃ b0 ∈ B, UniqueMul A B a0 b0

Tactic:
refine ⟨a1, ha1, b2, hb2, fun a b ha hb he => ?_⟩

Result:
case intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
G : Type u_1
inst✝¹ : Semigroup G
inst✝ : IsCancelMul G
h : ∀ {A : Finset G}, A.Nonempty → ∃ a1 ∈ A, ∃ a2 ∈ A, UniqueMul A A a1 a2
A B : Finset G
hA : A.Nonempty
hB : B.Nonempty
b1 : G
hb1 : b1 ∈ B
a1 : G
ha1 : a1 ∈ A
h1 : b1 * a1 ∈ B * A
b2 : G
hb2 : b2 ∈ B
a2 : G
ha2 : a2 ∈ A
h2 : b2 * a2 ∈ B * A
hu : UniqueMul (B * A) (B * A) (b1 * a1) (b2 * a2)
a b : G
ha : a ∈ A
hb : b ∈ B
he : a * b = a1 * b2
⊢ a = a1 ∧ b = b2