-- Lean 4 proof step from Mathlib.RingTheory.FinitePresentation (goal)
Goal:
R : Type w₁
A : Type w₂
B : Type w₃
inst : CommRing R
inst_1 : CommRing A
inst_2 : Algebra R A
inst_3 : CommRing B
inst_4 : Algebra R B
inst_5 : Algebra A B
inst_6 : IsScalarTower R A B
inst_7 : FinitePresentation R B
inst_8 : FiniteType R A
⊢ FinitePresentation A B
have obtain : ∀ (n : ℕ) (f : MvPolynomial (Fin n) R →ₐ[R] B)
  (hf : Surjective.{w₁ + 1, w₃ + 1} (α := MvPolynomial (Fin n) R) (β := B) (⇑f : (a : MvPolynomial (Fin n) R) → B))
  (s : Finset (MvPolynomial (Fin n) R))
  (hs :
    Eq (α := Ideal (MvPolynomial (Fin n) R)) (Ideal.span (↑s : Set (MvPolynomial (Fin n) R)))
      (RingHom.ker f.toRingHom)),
  FinitePresentation A B
Tactic: classical
  obtain ⟨n, f, hf, s, hs⟩ := FinitePresentation.out (R := R) (A := B)
  letI RX := MvPolynomial (Fin n) R
  letI AX := MvPolynomial (Fin n) A
  refine ⟨n, MvPolynomial.aeval (f ∘ X), ?_, ?_⟩
  · rw [← AlgHom.range_eq_top, ← Algebra.adjoin_range_eq_range_aeval,
      Set.range_comp f MvPolynomial.X, eq_top_iff, ← @adjoin_adjoin_of_tower R A B,
      adjoin_image, adjoin_range_X, Algebra.map_top, (AlgHom.range_eq_top _).mpr hf]
    exact fun {x} => subset_adjoin ⟨⟩
  · obtain ⟨t, ht⟩ := FiniteType.out (R := R) (A := A)
    have := fun i : t => hf (algebraMap A B i)
    choose t' ht' using this
    have ht'' : Algebra.adjoin R (algebraMap A AX '' t ∪ Set.range (X : _ → AX)) = ⊤ := by
      rw [adjoin_union_eq_adjoin_adjoin, ← Subalgebra.restrictScalars_top R (A := AX)
        (S := { x // x ∈ adjoin R ((algebraMap A AX) '' t) })]
      refine congrArg (Subalgebra.restrictScalars R) ?_
      rw [adjoin_algebraMap, ht]
      apply Subalgebra.restrictScalars_injective R
      rw [← adjoin_restrictScalars, adjoin_range_X, Subalgebra.restrictScalars_top,
        Subalgebra.restrictScalars_top]
    letI g : t → AX := fun x => MvPolynomial.C (x : A) - map (algebraMap R A) (t' x)
    refine ⟨s.image (map (algebraMap R A)) ∪ t.attach.image g, ?_⟩
    rw [Finset.coe_union, Finset.coe_image, Finset.coe_image, Finset.attach_eq_univ,
      Finset.coe_univ, Set.image_univ]
    let s₀ := (MvPolynomial.map (algebraMap R A)) '' s ∪ Set.range g
    let I := RingHom.ker (MvPolynomial.aeval (R := A) (f ∘ MvPolynomial.X))
    change Ideal.span s₀ = I
    have leI : Ideal.span ((MvPolynomial.map (algebraMap R A)) '' s ∪ Set.range g) ≤
      RingHom.ker (MvPolynomial.aeval (R := A) (f ∘ MvPolynomial.X)) := by
      rw [Ideal.span_le]
      rintro _ (⟨x, hx, rfl⟩ | ⟨⟨x, hx⟩, rfl⟩) <;>
      rw [SetLike.mem_coe, RingHom.mem_ker]
      · rw [MvPolynomial.aeval_map_algebraMap (R := R) (A := A), ← aeval_unique]
        have := Ideal.subset_span hx
        rwa [hs] at this
      · rw [map_sub, MvPolynomial.aeval_map_algebraMap, ← aeval_unique,
          MvPolynomial.aeval_C, ht', Subtype.coe_mk, sub_self]
    apply leI.antisymm
    intro x hx
    rw [RingHom.mem_ker] at hx
    let s₀ := (MvPolynomial.map (algebraMap R A)) '' ↑s ∪ Set.range g
    change x ∈ Ideal.span s₀
    have : x ∈ (MvPolynomial.map (algebraMap R A) : _ →+* AX).range.toAddSubmonoid ⊔
      (Ideal.span s₀).toAddSubmonoid := by
      have : x ∈ (⊤ : Subalgebra R AX) := trivial
      rw [← ht''] at this
      refine adjoin_induction ?_ ?_ ?_ ?_ this
      · rintro _ (⟨x, hx, rfl⟩ | ⟨i, rfl⟩)
        · rw [MvPolynomial.algebraMap_eq, ← sub_add_cancel (MvPolynomial.C x)
            (map (algebraMap R A) (t' ⟨x, hx⟩)), add_comm]
          apply AddSubmonoid.add_mem_sup
          · exact Set.mem_range_self _
          · apply Ideal.subset_span
            apply Set.mem_union_right
            exact Set.mem_range_self _
        · apply AddSubmonoid.mem_sup_left
          exact ⟨X i, map_X _ _⟩
      · intro r
        apply AddSubmonoid.mem_sup_left
        exact ⟨C r, map_C _ _⟩
      · intro _ _ _ _ h₁ h₂
        exact add_mem h₁ h₂
      · intro x₁ x₂ _ _ h₁ h₂
        obtain ⟨_, ⟨p₁, rfl⟩, q₁, hq₁, rfl⟩ := AddSubmonoid.mem_sup.mp h₁
        obtain ⟨_, ⟨p₂, rfl⟩, q₂, hq₂, rfl⟩ := AddSubmonoid.mem_sup.mp h₂
        rw [add_mul, mul_add, add_assoc, ← map_mul]
        apply AddSubmonoid.add_mem_sup
        · exact Set.mem_range_self _
        · refine add_mem (Ideal.mul_mem_left _ _ hq₂) (Ideal.mul_mem_right _ _ hq₁)
    obtain ⟨_, ⟨p, rfl⟩, q, hq, rfl⟩ := AddSubmonoid.mem_sup.mp this
    rw [map_add, aeval_map_algebraMap, ← aeval_unique, show MvPolynomial.aeval (f ∘ X) q = 0
      from leI hq, add_zero] at hx
    suffices Ideal.span (s : Set RX) ≤ (Ideal.span s₀).comap (MvPolynomial.map <| algebraMap R A) by
      refine add_mem ?_ hq
      rw [hs] at this
      exact this hx
    rw [Ideal.span_le]
    intro x hx
    apply Ideal.subset_span
    apply Set.mem_union_left
    exact Set.mem_image_of_mem _ hx