/-- In a noncommutative ordered `*`-algebra over ℝ,
Tsirelson's bound for a CHSH tuple (A₀, A₁, B₀, B₁) is
`A₀ * B₀ + A₀ * B₁ + A₁ * B₀ - A₁ * B₁ ≤ 2^(3/2) • 1`.

We prove this by providing an explicit sum-of-squares decomposition
of the difference.

(We could work over `ℤ[2^(1/2), 2^(-1/2)]` if we really wanted to!)
-/
theorem tsirelson_inequality [OrderedRing R] [StarOrderedRing R] [Algebra ℝ R] [OrderedSMul ℝ R]
    [StarModule ℝ R] (A₀ A₁ B₀ B₁ : R) (T : IsCHSHTuple A₀ A₁ B₀ B₁) :
    A₀ * B₀ + A₀ * B₁ + A₁ * B₀ - A₁ * B₁ ≤ √2 ^ 3 • (1 : R) := by
  -- abel will create `ℤ` multiplication. We will `simp` them away to `ℝ` multiplication.
  have M : ∀ (m : ℤ) (a : ℝ) (x : R), m • a • x = ((m : ℝ) * a) • x := fun m a x => by
    rw [zsmul_eq_smul_cast ℝ, ← mul_smul]
  let P := √2⁻¹ • (A₁ + A₀) - B₀
  let Q := √2⁻¹ • (A₁ - A₀) + B₁
  have w : √2 ^ 3 • (1 : R) - A₀ * B₀ - A₀ * B₁ - A₁ * B₀ + A₁ * B₁ = √2⁻¹ • (P ^ 2 + Q ^ 2) := by
    dsimp
    -- distribute out all the powers and products appearing on the RHS
    simp only [sq, sub_mul, mul_sub, add_mul, mul_add, smul_add, smul_sub]
    -- pull all coefficients out to the front, and combine `√2`s where possible
    simp only [Algebra.mul_smul_comm, Algebra.smul_mul_assoc, ← mul_smul, sqrt_two_inv_mul_self]
    -- replace Aᵢ * Aᵢ = 1 and Bᵢ * Bᵢ = 1
    simp only [← sq, T.A₀_inv, T.A₁_inv, T.B₀_inv, T.B₁_inv]
    -- move Aᵢ to the left of Bᵢ
    simp only [← T.A₀B₀_commutes, ← T.A₀B₁_commutes, ← T.A₁B₀_commutes, ← T.A₁B₁_commutes]
    -- collect terms, simplify coefficients, and collect terms again:
    abel_nf
    -- all terms coincide, but the last one. Simplify all other terms
    simp only [M]
    simp only [neg_mul, one_mul, mul_inv_cancel_of_invertible, Int.cast_one, add_assoc, add_comm,
      add_left_comm, one_smul, Int.cast_neg, neg_smul, Int.int_cast_ofNat]
    simp only [← add_assoc, ← add_smul]
    -- just look at the coefficients now:
    congr
    exact mul_left_cancel₀ (by norm_num) tsirelson_inequality_aux
  have pos : 0 ≤ √2⁻¹ • (P ^ 2 + Q ^ 2) := by
    have P_sa : star P = P := by
      simp only [star_smul, star_add, star_sub, star_id_of_comm, T.A₀_sa, T.A₁_sa, T.B₀_sa, T.B₁_sa]
    have Q_sa : star Q = Q := by
      simp only [star_smul, star_add, star_sub, star_id_of_comm, T.A₀_sa, T.A₁_sa, T.B₀_sa, T.B₁_sa]
    have P2_nonneg : 0 ≤ P ^ 2 := by
      rw [sq]
      conv =>
        congr
        skip
        congr
        rw [← P_sa]
      convert (star_mul_self_nonneg P)
    have Q2_nonneg : 0 ≤ Q ^ 2 := by
      rw [sq]
      conv =>
        congr
        skip
        congr
        rw [← Q_sa]
      convert (star_mul_self_nonneg Q)
    convert smul_le_smul_of_nonneg (add_nonneg P2_nonneg Q2_nonneg)
        (le_of_lt (show 0 < √2⁻¹ by norm_num))
    -- `norm_num` can't directly show `0 ≤ √2⁻¹`
    simp
  apply le_of_sub_nonneg
  

Proof state:
case a
R : Type u
inst✝⁴ : OrderedRing R
inst✝³ : StarOrderedRing R
inst✝² : Algebra ℝ R
inst✝¹ : OrderedSMul ℝ R
inst✝ : StarModule ℝ R
A₀ A₁ B₀ B₁ : R
T : IsCHSHTuple A₀ A₁ B₀ B₁
M : ∀ (m : ℤ) (a : ℝ) (x : R), m • a • x = (↑m * a) • x
P : R := (Real.sqrt 2)⁻¹ • (A₁ + A₀) - B₀
Q : R := (Real.sqrt 2)⁻¹ • (A₁ - A₀) + B₁
w : Real.sqrt 2 ^ 3 • 1 - A₀ * B₀ - A₀ * B₁ - A₁ * B₀ + A₁ * B₁ = (Real.sqrt 2)⁻¹ • (P ^ 2 + Q ^ 2)
pos : 0 ≤ (Real.sqrt 2)⁻¹ • (P ^ 2 + Q ^ 2)
⊢ 0 ≤ Real.sqrt 2 ^ 3 • 1 - (A₀ * B₀ + A₀ * B₁ + A₁ * B₀ - A₁ * B₁)

Next tactic:
simpa only [sub_add_eq_sub_sub, ← sub_add, w, Nat.cast_zero] using pos