-- Lean 4 proof step from Mathlib.RingTheory.RingHom.Finite (goal)
Goal:
rw rw₁ rw₂ : OfLocalizationFiniteSpan @Finite
rw₃ :
  ∀ ⦃R S : Type u_5⦄ [inst : CommRing R] [inst_1 : CommRing S] (f : R →+* S) (s : Finset R)
    (hs : Eq (α := Ideal R) (Ideal.span (↑s : Set R)) ⊤)
    (H : ∀ (r : ↥s), Finite (A := Localization.Away (↑r : R)) (Localization.awayMap f (↑r : R))), f.Finite
rw₄ rw₅ :
  ∀ ⦃R S : Type u_5⦄ [inst : CommRing R] [inst_1 : CommRing S] (f : R →+* S) (s : Finset R)
    (hs : Eq (α := Ideal R) (Ideal.span (↑s : Set R)) ⊤)
    (H : ∀ (r : ↥s), Finite (A := Localization.Away (↑r : R)) (Localization.awayMap f (↑r : R))), f.Finite
rw₆ :
  ∀ ⦃R S : Type u_5⦄ [inst : CommRing R] [inst_1 : CommRing S] (f : R →+* S) (s : Finset R)
    (hs : Eq (α := Ideal R) (Ideal.span (↑s : Set R)) ⊤)
    (H : ∀ (r : ↥s), Finite (A := Localization.Away (↑r : R)) (Localization.awayMap f (↑r : R))),
    let this := f.toAlgebra;
    ∀
      (this :
        ∀ (r : ↥s),
          IsLocalization (Submonoid.map (F := R →+* S) (algebraMap R S) (Submonoid.powers (↑r : R)))
            (Localization.Away (M := S) ((f : (a : R) → S) (↑r : R) : S))),
      f.Finite
⊢ OfLocalizationSpan @Finite
have rw₇ : ∀ ⦃R S : Type u_5⦄ [inst : CommRing R] [inst_1 : CommRing S] (f : R →+* S) (s : Finset R)
  (hs : Eq (α := Ideal R) (Ideal.span (↑s : Set R)) ⊤)
  (H : ∀ (r : ↥s), Finite (A := Localization.Away (↑r : R)) (Localization.awayMap f (↑r : R))),
  let this := f.toAlgebra;
  let this_1 :
    (r : ↥s) → Algebra (Localization.Away (↑r : R)) (Localization.Away (M := S) ((f : (a : R) → S) (↑r : R) : S)) :=
    fun (r : ↥s) ↦ toAlgebra (Localization.awayMap f (↑r : R));
  ∀
    (this_2 :
      ∀ (r : ↥s),
        IsLocalization (Submonoid.map (F := R →+* S) (algebraMap R S) (Submonoid.powers (↑r : R)))
          (Localization.Away (M := S) ((f : (a : R) → S) (↑r : R) : S)))
    (this :
      ∀ (r : ↥s),
        IsScalarTower R (Localization.Away (↑r : R)) (Localization.Away (M := S) ((f : (a : R) → S) (↑r : R) : S))),
    f.Finite
Tactic: classical
  rw [RingHom.ofLocalizationSpan_iff_finite]
  introv R hs H
  -- We first setup the instances
  letI := f.toAlgebra
  letI := fun r : s => (Localization.awayMap f r).toAlgebra
  have (r : s) : IsLocalization ((Submonoid.powers (r : R)).map (algebraMap R S))
      (Localization.Away (f r)) := by
    rw [Submonoid.map_powers]; exact Localization.isLocalization
  haveI : ∀ r : s, IsScalarTower R (Localization.Away (r : R)) (Localization.Away (f r)) :=
    fun r => IsScalarTower.of_algebraMap_eq'
      (IsLocalization.map_comp (Submonoid.powers (r : R)).le_comap_map).symm
  -- By the hypothesis, we may find a finite generating set for each `Sᵣ`. This set can then be
  -- lifted into `R` by multiplying a sufficiently large power of `r`. I claim that the union of
  -- these generates `S`.
  constructor
  replace H := fun r => (H r).1
  choose s₁ s₂ using H
  let sf := fun x : s => IsLocalization.finsetIntegerMultiple (Submonoid.powers (f x)) (s₁ x)
  use s.attach.biUnion sf
  rw [Submodule.span_attach_biUnion, eq_top_iff]
  -- It suffices to show that `r ^ n • x ∈ span T` for each `r : s`, since `{ r ^ n }` spans `R`.
  -- This then follows from the fact that each `x : R` is a linear combination of the generating set
  -- of `Sᵣ`. By multiplying a sufficiently large power of `r`, we can cancel out the `r`s in the
  -- denominators of both the generating set and the coefficients.
  rintro x -
  apply Submodule.mem_of_span_eq_top_of_smul_pow_mem _ (s : Set R) hs _ _
  intro r
  obtain ⟨⟨_, n₁, rfl⟩, hn₁⟩ :=
    multiple_mem_span_of_mem_localization_span (Submonoid.powers (r : R))
      (Localization.Away (r : R)) (s₁ r : Set (Localization.Away (f r))) (algebraMap S _ x)
      (by rw [s₂ r]; trivial)
  dsimp only at hn₁
  rw [Submonoid.smul_def, Algebra.smul_def, IsScalarTower.algebraMap_apply R S, ← map_mul] at hn₁
  obtain ⟨⟨_, n₂, rfl⟩, hn₂⟩ :=
    IsLocalization.smul_mem_finsetIntegerMultiple_span (Submonoid.powers (r : R))
      (Localization.Away (f r)) _ (s₁ r) hn₁
  rw [Submonoid.smul_def, ← Algebra.smul_def, smul_smul, ← pow_add] at hn₂
  simp_rw [Submonoid.map_powers] at hn₂
  use n₂ + n₁
  exact le_iSup (fun x : s => Submodule.span R (sf x : Set S)) r hn₂