/-- This is used to prove the strictly stronger `ker_fg_of_surjective`. Use it instead. -/
theorem ker_fg_of_mvPolynomial {n : ℕ} (f : MvPolynomial (Fin n) R →ₐ[R] A)
    (hf : Function.Surjective f) (hfp : FinitePresentation R A) : f.toRingHom.ker.FG := by
  classical
    obtain ⟨m, f', hf', s, hs⟩ := hfp
    let RXn := MvPolynomial (Fin n) R
    let RXm := MvPolynomial (Fin m) R
    have := fun i : Fin n => hf' (f <| X i)
    choose g hg using this
    have := fun i : Fin m => hf (f' <| X i)
    choose h hh using this
    let aeval_h : RXm →ₐ[R] RXn := aeval h
    let g' : Fin n → RXn := fun i => X i - aeval_h (g i)
    refine' ⟨Finset.univ.image g' ∪ s.image aeval_h, _⟩
    simp only [Finset.coe_image, Finset.coe_union, Finset.coe_univ, Set.image_univ]
    have hh' : ∀ x, f (aeval_h x) = f' x := by
      intro x
      rw [← f.coe_toRingHom, map_aeval]
      simp_rw [AlgHom.coe_toRingHom, hh]
      rw [AlgHom.comp_algebraMap, ← aeval_eq_eval₂Hom,
        -- Porting note: added line below
        ← funext fun i => Function.comp_apply (f := ↑f') (g := MvPolynomial.X),
        ← aeval_unique]
    let s' := Set.range g' ∪ aeval_h '' s
    have leI : Ideal.span s' ≤ RingHom.ker f.toRingHom := by
      rw [Ideal.span_le]
      rintro _ (⟨i, rfl⟩ | ⟨x, hx, rfl⟩)
      · change f (g' i) = 0
        rw [map_sub, ← hg, hh', sub_self]
      · change f (aeval_h x) = 0
        rw [hh']
        change x ∈ RingHom.ker f'.toRingHom
        rw [← hs]
        exact Ideal.subset_span hx
    apply leI.antisymm
    intro x hx
    have : x ∈ aeval_h.range.toAddSubmonoid ⊔ (Ideal.span s').toAddSubmonoid := by
      have : x ∈ adjoin R (Set.range X : Set RXn) := by
        rw [adjoin_range_X]
        trivial
      refine adjoin_induction this ?_ ?_ ?_ ?_
      · rintro _ ⟨i, rfl⟩
        rw [← sub_add_cancel (X i) (aeval h (g i)), add_comm]
        apply AddSubmonoid.add_mem_sup
        · exact Set.mem_range_self _
        · apply Submodule.subset_span
          apply Set.mem_union_left
          exact Set.mem_range_self _
      · intro r
        apply AddSubmonoid.mem_sup_left
        exact ⟨C r, aeval_C _ _⟩
      · intro _ _ h₁ h₂
        exact add_mem h₁ h₂
      · intro p₁ p₂ h₁ h₂
        obtain ⟨_, ⟨x₁, rfl⟩, y₁, hy₁, rfl⟩ := AddSubmonoid.mem_sup.mp h₁
        obtain ⟨_, ⟨x₂, rfl⟩, y₂, hy₂, rfl⟩ := AddSubmonoid.mem_sup.mp h₂
        rw [mul_add, add_mul, add_assoc, ← map_mul]
        apply AddSubmonoid.add_mem_sup
        · exact Set.mem_range_self _
        · exact add_mem (Ideal.mul_mem_right _ _ hy₁) (Ideal.mul_mem_left _ _ hy₂)
    obtain ⟨_, ⟨x, rfl⟩, y, hy, rfl⟩ := AddSubmonoid.mem_sup.mp this
    refine' add_mem _ hy
    

Proof state:
case intro.intro.intro.intro.intro.intro.intro.intro.intro
R : Type w₁
A : Type w₂
B : Type w₃
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : CommRing B
inst✝ : Algebra R B
n : ℕ
f : MvPolynomial (Fin n) R →ₐ[R] A
hf : Surjective ⇑f
m : ℕ
f' : MvPolynomial (Fin m) R →ₐ[R] A
hf' : Surjective ⇑f'
s : Finset (MvPolynomial (Fin m) R)
hs : Ideal.span ↑s = RingHom.ker ↑f'
RXn : Type w₁ := MvPolynomial (Fin n) R
RXm : Type w₁ := MvPolynomial (Fin m) R
g : Fin n → MvPolynomial (Fin m) R
hg : ∀ (i : Fin n), f' (g i) = f (MvPolynomial.X i)
h : Fin m → MvPolynomial (Fin n) R
hh : ∀ (i : Fin m), f (h i) = f' (MvPolynomial.X i)
aeval_h : RXm →ₐ[R] RXn := MvPolynomial.aeval h
g' : Fin n → RXn := fun i => MvPolynomial.X i - aeval_h (g i)
hh' : ∀ (x : RXm), f (aeval_h x) = f' x
s' : Set RXn := Set.range g' ∪ ⇑aeval_h '' ↑s
leI : Ideal.span s' ≤ RingHom.ker ↑f
x : RXm
y : RXn
hy : y ∈ (Ideal.span s').toAddSubmonoid
hx : ↑aeval_h x + y ∈ RingHom.ker ↑f
this : ↑aeval_h x + y ∈ Subalgebra.toAddSubmonoid (AlgHom.range aeval_h) ⊔ (Ideal.span s').toAddSubmonoid
⊢ ↑aeval_h x ∈ Ideal.span s'

Next tactic:
simp only [RingHom.mem_ker, AlgHom.toRingHom_eq_coe, AlgHom.coe_toRingHom, map_add,
      show f y = 0 from leI hy, add_zero, hh'] at hx