theorem normalize_finite_fraction_representation (U : Opens (PrimeSpectrum.Top R))
    (s : (structureSheaf R).1.obj (op U)) {ι : Type*} (t : Finset ι) (a h : ι → R)
    (iDh : ∀ i : ι, PrimeSpectrum.basicOpen (h i) ⟶ U)
    (h_cover : U ≤ ⨆ i ∈ t, PrimeSpectrum.basicOpen (h i))
    (hs :
      ∀ i : ι,
        (const R (a i) (h i) (PrimeSpectrum.basicOpen (h i)) fun y hy => hy) =
          (structureSheaf R).1.map (iDh i).op s) :
    ∃ (a' h' : ι → R) (iDh' : ∀ i : ι, PrimeSpectrum.basicOpen (h' i) ⟶ U),
      (U ≤ ⨆ i ∈ t, PrimeSpectrum.basicOpen (h' i)) ∧
        (∀ (i) (_ : i ∈ t) (j) (_ : j ∈ t), a' i * h' j = h' i * a' j) ∧
          ∀ i ∈ t,
            (structureSheaf R).1.map (iDh' i).op s =
              const R (a' i) (h' i) (PrimeSpectrum.basicOpen (h' i)) fun y hy => hy := by
  -- First we show that the fractions `(a i * h j) / (h i * h j)` and `(h i * a j) / (h i * h j)`
  -- coincide in the localization of `R` at `h i * h j`
  have fractions_eq :
    ∀ i j : ι,
      IsLocalization.mk' (Localization.Away (h i * h j))
        (a i * h j) ⟨h i * h j, Submonoid.mem_powers _⟩ =
      IsLocalization.mk' _ (h i * a j) ⟨h i * h j, Submonoid.mem_powers _⟩ := by
    intro i j
    let D := PrimeSpectrum.basicOpen (h i * h j)
    let iDi : D ⟶ PrimeSpectrum.basicOpen (h i) := homOfLE (PrimeSpectrum.basicOpen_mul_le_left _ _)
    let iDj : D ⟶ PrimeSpectrum.basicOpen (h j) :=
      homOfLE (PrimeSpectrum.basicOpen_mul_le_right _ _)
    -- Crucially, we need injectivity of `toBasicOpen`
    apply toBasicOpen_injective R (h i * h j)
    rw [toBasicOpen_mk', toBasicOpen_mk']
    simp only []
    -- Here, both sides of the equation are equal to a restriction of `s`
    trans
    convert congr_arg ((structureSheaf R).1.map iDj.op) (hs j).symm using 1
    convert congr_arg ((structureSheaf R).1.map iDi.op) (hs i) using 1; swap
    all_goals rw [res_const]; apply const_ext; ring
    -- The remaining two goals were generated during the rewrite of `res_const`
    -- These can be solved immediately
    exacts [PrimeSpectrum.basicOpen_mul_le_right _ _, PrimeSpectrum.basicOpen_mul_le_left _ _]
  -- From the equality in the localization, we obtain for each `(i,j)` some power `(h i * h j) ^ n`
  -- which equalizes `a i * h j` and `h i * a j`
  have exists_power :
    ∀ i j : ι, ∃ n : ℕ, a i * h j * (h i * h j) ^ n = h i * a j * (h i * h j) ^ n := by
    intro i j
    obtain ⟨⟨c, n, rfl⟩, hc⟩ := IsLocalization.eq.mp (fractions_eq i j)
    use n + 1
    rw [pow_succ]
    dsimp at hc
    convert hc using 1 <;> ring
  let n := fun p : ι × ι => (exists_power p.1 p.2).choose
  

Proof state:
R : Type u
inst✝ : CommRing R
U : Opens ↑(PrimeSpectrum.Top R)
s : ↑((structureSheaf R).val.obj (op U))
ι : Type u_1
t : Finset ι
a h : ι → R
iDh : (i : ι) → PrimeSpectrum.basicOpen (h i) ⟶ U
h_cover : U ≤ ⨆ i ∈ t, PrimeSpectrum.basicOpen (h i)
hs :
  ∀ (i : ι),
    const R (a i) (h i) (PrimeSpectrum.basicOpen (h i))
        (_ : ∀ y ∈ PrimeSpectrum.basicOpen (h i), y ∈ PrimeSpectrum.basicOpen (h i)) =
      ((structureSheaf R).val.map (iDh i).op) s
fractions_eq :
  ∀ (i j : ι),
    IsLocalization.mk' (Localization.Away (h i * h j)) (a i * h j)
        { val := h i * h j, property := (_ : h i * h j ∈ Submonoid.powers (h i * h j)) } =
      IsLocalization.mk' (Localization.Away (h i * h j)) (h i * a j)
        { val := h i * h j, property := (_ : h i * h j ∈ Submonoid.powers (h i * h j)) }
exists_power : ∀ (i j : ι), ∃ n, a i * h j * (h i * h j) ^ n = h i * a j * (h i * h j) ^ n
n : ι × ι → ℕ :=
  fun p => Exists.choose (_ : ∃ n, a p.1 * h p.2 * (h p.1 * h p.2) ^ n = h p.1 * a p.2 * (h p.1 * h p.2) ^ n)
⊢ ∃ a' h' iDh',
    U ≤ ⨆ i ∈ t, PrimeSpectrum.basicOpen (h' i) ∧
      (∀ i ∈ t, ∀ j ∈ t, a' i * h' j = h' i * a' j) ∧
        ∀ i ∈ t,
          ((structureSheaf R).val.map (iDh' i).op) s =
            const R (a' i) (h' i) (PrimeSpectrum.basicOpen (h' i))
              (_ : ∀ y ∈ PrimeSpectrum.basicOpen (h' i), y ∈ PrimeSpectrum.basicOpen (h' i))

Next tactic:
have n_spec := fun p : ι × ι => (exists_power p.fst p.snd).choose_spec