-- Lean 4 proof step from Mathlib.RingTheory.Smooth.NoetherianDescent (hypothesis)
Goal:
R : Type u_1
inst : CommRing R
A : Type u
B : Type u_2
inst_1 : CommRing A
inst_2 : Algebra R A
inst_3 : CommRing B
inst_4 : Algebra A B
inst_5 : Smooth A B
P : Presentation A B (Fin (Presentation.ofFinitePresentationVars A B))
  (Fin (Presentation.ofFinitePresentationRels A B)) :=
  Presentation.ofFinitePresentation A B
f : P.Ring →ₐ[A] B := IsScalarTower.toAlgHom A P.Ring B
hkerf : Eq (α := Ideal P.Ring) (RingHom.ker f) (Ideal.span (Set.range P.relation))
σ :
  B →ₐ[A]
    MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸
      HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2
hsig : f.kerSquareLift.comp σ = AlgHom.id A B
h :
  (i : Fin (Presentation.ofFinitePresentationVars A B)) →
    MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A
hh :
  ∀ (i : Fin (Presentation.ofFinitePresentationVars A B)),
    Eq (α :=
      MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸
        HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2)
      ((Ideal.Quotient.mk (HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2) :
            (a : MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A) →
              MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸ RingHom.ker f ^ 2)
          (h i) :
        MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸
          HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2)
      ((σ : (a : B) → MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸ RingHom.ker f ^ 2) (P.val i) :
        MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸
          HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2)
hdiag :
  AlgHom.comp (Ideal.Quotient.mkₐ A (HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2)) (aeval h) =
    σ.comp (aeval P.val)
⊢ ∃ (A₀ : Subalgebra R A) (B₀ : Type u) (x : CommRing B₀) (x_1 : Algebra (↥A₀) B₀),
    A₀.FG ∧ Smooth (↥A₀) B₀ ∧ Nonempty (B ≃ₐ[A] A ⊗[↥A₀] B₀)
have this : ∀ (j : Fin (Presentation.ofFinitePresentationRels A B)),
  Eq (α :=
    MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸
      HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2)
    ((Ideal.Quotient.mk (HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2) :
          (a : MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A) →
            MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸ RingHom.ker f ^ 2)
        ((aeval h :
            (a : MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A) →
              MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A)
          (P.relation j)) :
      MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸
        HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2)
    (0 :
      MvPolynomial (Fin (Presentation.ofFinitePresentationVars A B)) A ⧸
        HPow.hPow (α := Ideal P.Ring) (β := ℕ) (RingHom.ker f) 2)
Tactic: have (j : _) : Ideal.Quotient.mk (RingHom.ker f ^ 2) (aeval h (P.relation j)) = 0 := by
    suffices ho : σ (aeval P.val (P.relation j)) = 0 by
      convert! ho
      exact congr($hdiag _)
    simp