-- Mathlib/RingTheory/Localization/Away/Basic.lean
theorem existsUnique_algebraMap_eq_of_span_eq_top (s : Set R) (span_eq : Ideal.span s = ⊤)
    (f : Π a : s, Away a.1) (h : ∀ a b : s,
      Away.awayToAwayRight (P := Away (a * b : R)) a.1 b (f a) = Away.awayToAwayLeft b.1 a (f b)) :
    ∃! r : R, ∀ a : s, algebraMap R _ r = f a

Proof state:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝ : ↑↑s → R
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝ a)
N : ℕ := s.attach.sup n
r : ↑↑s → R := fun a => ↑a ^ (N - n a) * r✝ a
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Tactic:
have eq a : f a * algebraMap R _ (a ^ N) = algebraMap R _ (r a) := by
  rw [map_mul, ← eq, mul_left_comm, ← map_pow, ← map_mul, ← pow_add,
    Nat.sub_add_cancel (Finset.le_sup <| s.mem_attach a)]

Result:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝ : ↑↑s → R
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝ a)
N : ℕ := s.attach.sup n
r : ↑↑s → R := fun a => ↑a ^ (N - n a) * r✝ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N) = (algebraMap R (Away ↑a)) (r a)
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Proof state:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝¹ : ↑↑s → R
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝¹ a)
N✝ : ℕ := s.attach.sup n
r✝ : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝¹ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r✝ a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r✝ b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2 : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r✝ b * ↑a ^ N✝)
N : ℕ := N' + N✝
r : ↑↑s → R := fun a => ↑a ^ N' * r✝ a
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Tactic:
have eq a : f a * algebraMap R _ (a ^ N) = algebraMap R _ (r a) := by
  rw [map_mul, ← eq, mul_left_comm, ← map_mul, ← pow_add]

Result:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝¹ : ↑↑s → R
eq✝¹ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝¹ a)
N✝ : ℕ := s.attach.sup n
r✝ : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝¹ a
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r✝ a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r✝ b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2 : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r✝ b * ↑a ^ N✝)
N : ℕ := N' + N✝
r : ↑↑s → R := fun a => ↑a ^ N' * r✝ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N) = (algebraMap R (Away ↑a)) (r a)
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Proof state:
case inr
R : Type u_1
inst✝ : CommSemiring R
s : Set R
f : (a : ↑s) → Away ↑a
h : ∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span s
this :
  ∀ (s : Set R) (f : (a : ↑s) → Away ↑a),
    (∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)) →
      1 ∈ Ideal.span s → (∃ t, ↑t = s) → ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a
finset_eq : ¬∃ t, ↑t = s
⊢ ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a

Tactic:
have ⟨t, hts, mem⟩ := Submodule.mem_span_finite_of_mem_span mem

Result:
case inr
R : Type u_1
inst✝ : CommSemiring R
s : Set R
f : (a : ↑s) → Away ↑a
h : ∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem✝ : 1 ∈ Ideal.span s
this :
  ∀ (s : Set R) (f : (a : ↑s) → Away ↑a),
    (∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)) →
      1 ∈ Ideal.span s → (∃ t, ↑t = s) → ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a
finset_eq : ¬∃ t, ↑t = s
t : Finset R
hts : ↑t ⊆ s
mem : 1 ∈ Submodule.span R ↑t
⊢ ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a

Proof state:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝¹ : ↑↑s → R
eq✝¹ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝¹ a)
N✝ : ℕ := s.attach.sup n
r✝ : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝¹ a
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r✝ a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r✝ b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2 : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r✝ b * ↑a ^ N✝)
N : ℕ := N' + N✝
r : ↑↑s → R := fun a => ↑a ^ N' * r✝ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N) = (algebraMap R (Away ↑a)) (r a)
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Tactic:
have eq2 a b : r a * b ^ N = r b * a ^ N := by
  rw [pow_add, mul_mul_mul_comm, ← mul_pow, eq2,
    mul_comm a.1, mul_pow, mul_mul_mul_comm, ← pow_add]

Result:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝¹ : ↑↑s → R
eq✝¹ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝¹ a)
N✝ : ℕ := s.attach.sup n
r✝ : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝¹ a
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r✝ a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r✝ b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2✝ : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r✝ b * ↑a ^ N✝)
N : ℕ := N' + N✝
r : ↑↑s → R := fun a => ↑a ^ N' * r✝ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N) = (algebraMap R (Away ↑a)) (r a)
eq2 : ∀ (a b : ↑↑s), r a * ↑b ^ N = r b * ↑a ^ N
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a