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
  have n_spec := fun p : ι × ι => (exists_power p.fst p.snd).choose_spec
  -- We need one power `(h i * h j) ^ N` that works for *all* pairs `(i,j)`
  -- Since there are only finitely many indices involved, we can pick the supremum.
  let N := (t ×ˢ t).sup n
  have basic_opens_eq : ∀ i : ι, PrimeSpectrum.basicOpen (h i ^ (N + 1)) =
    PrimeSpectrum.basicOpen (h i) := fun i => PrimeSpectrum.basicOpen_pow _ _ (by linarith)
  -- Expanding the fraction `a i / h i` by the power `(h i) ^ n` gives the desired normalization
  refine'
    ⟨fun i => a i * h i ^ N, fun i => h i ^ (N + 1), fun i => eqToHom (basic_opens_eq i) ≫ iDh i,
      _, _, _⟩
  · simpa only [basic_opens_eq] using h_cover
  · intro i hi j hj
    -- Here we need to show that our new fractions `a i / h i` satisfy the normalization condition
    -- Of course, the power `N` we used to expand the fractions might be bigger than the power
    -- `n (i, j)` which was originally chosen. We denote their difference by `k`
    have n_le_N : n (i, j) ≤ N := Finset.le_sup (Finset.mem_product.mpr ⟨hi, hj⟩)
    cases' Nat.le.dest n_le_N with k hk
    simp only [← hk, pow_add, pow_one]
    -- To accommodate for the difference `k`, we multiply both sides of the equation `n_spec (i, j)`
    -- by `(h i * h j) ^ k`
    convert congr_arg (fun z => z * (h i * h j) ^ k) (n_spec (i, j)) using 1 <;>
      · simp only [mul_pow]; ring
  -- Lastly, we need to show that the new fractions still represent our original `s`
  intro i _
  rw [op_comp, Functor.map_comp]
  -- Porting note : `comp_apply` can't be rewritten, so use a change
  change (structureSheaf R).1.map (eqToHom (basic_opens_eq _)).op
    ((structureSheaf R).1.map (iDh i).op s) = _
  

Proof state:
case refine'_3
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)
n_spec :
  ∀ (p : ι × ι),
    a p.1 * h p.2 *
        (h p.1 * h p.2) ^
          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) =
      h p.1 * a p.2 *
        (h p.1 * h p.2) ^
          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)
N : ℕ := Finset.sup (t ×ˢ t) n
basic_opens_eq : ∀ (i : ι), PrimeSpectrum.basicOpen (h i ^ (N + 1)) = PrimeSpectrum.basicOpen (h i)
i : ι
a✝ : i ∈ t
⊢ ((structureSheaf R).val.map
        (eqToHom (_ : PrimeSpectrum.basicOpen (h i ^ (N + 1)) = PrimeSpectrum.basicOpen (h i))).op)
      (((structureSheaf R).val.map (iDh i).op) s) =
    const R ((fun i => a i * h i ^ N) i) ((fun i => h i ^ (N + 1)) i)
      (PrimeSpectrum.basicOpen ((fun i => h i ^ (N + 1)) i))
      (_ :
        ∀ y ∈ PrimeSpectrum.basicOpen ((fun i => h i ^ (N + 1)) i),
          y ∈ PrimeSpectrum.basicOpen ((fun i => h i ^ (N + 1)) i))

Next tactic:
rw [← hs, res_const]