-- Lean 4 proof step from Mathlib.RingTheory.Spectrum.Prime.ChevalleyComplexity (goal)
Goal:
R : Type u_2
inst : CommRing R
n_1 n : ℕ
IH :
  ∀ {M : Submodule ℤ R} (hM : 1 ∈ M) (k : ℕ) (d : Multiset (Fin n)) (S : ConstructibleSetData (MvPolynomial (Fin n) R))
    (hSn : ∀ C ∈ S, C.n ≤ k)
    (hS :
      ∀ C ∈ S,
        ∀ (j : Fin C.n),
          Membership.mem (γ := Submodule ℤ (MvPolynomial (Fin n) R))
            (coeffsIn (Fin n) M ⊓ Submodule.restrictScalars ℤ (degreesLE R (Fin n) d)) (C.g j)),
    ∃ (T : ConstructibleSetData R),
      comap MvPolynomial.C '' S.toSet = T.toSet ∧
        ∀ C ∈ T,
          C.n ≤ numBound k (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val d)) n ∧
            ∀ (i : Fin C.n), C.g i ∈ M ^ degBound k (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val d)) n
M : Submodule ℤ R
hM : 1 ∈ M
k : ℕ
d : Multiset (Fin (n + 1))
S : ConstructibleSetData (MvPolynomial (Fin (n + 1)) R)
hSn : ∀ C ∈ S, C.n ≤ k
hS :
  ∀ C ∈ S,
    ∀ (j : Fin C.n),
      Membership.mem (γ := Submodule ℤ (MvPolynomial (Fin (n + 1)) R))
        (coeffsIn (Fin (n + 1)) M ⊓ Submodule.restrictScalars ℤ (degreesLE R (Fin (n + 1)) d)) (C.g j)
e : MvPolynomial (Fin (n + 1)) R ≃ₐ[R] (MvPolynomial (Fin n) R)[X] := MvPolynomial.finSuccEquiv R n
S' : ConstructibleSetData (MvPolynomial (Fin n) R)[X] := ConstructibleSetData.map e.toRingEquiv.toRingHom S
hS' : S'.degBound ≤ k * (1 + Multiset.count 0 d)
B : Multiset (Fin n) :=
  (Finsupp.toMultiset : (a : Fin n →₀ ℕ) → Multiset (Fin n))
    (Finsupp.comapDomain Fin.succ ((Multiset.toFinsupp : (a : Multiset (Fin (n + 1))) → Fin (n + 1) →₀ ℕ) d) ⋯)
⊢ ∃ (T : ConstructibleSetData R),
    comap MvPolynomial.C '' S.toSet = T.toSet ∧
      ∀ C ∈ T,
        C.n ≤ numBound k (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val d)) (n + 1) ∧
          ∀ (i : Fin C.n), C.g i ∈ M ^ degBound k (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val d)) (n + 1)
have succ : ∀ (T : ConstructibleSetData (MvPolynomial (Fin n) R)) (hT₁ : comap Polynomial.C '' S'.toSet = T.toSet)
  (hT₂ :
    ∀ C ∈ T,
      C.n ≤ S'.degBound ∧
        ∀ (i : Fin C.n),
          C.g i ∈
            HPow.hPow (α := Submodule ℤ (MvPolynomial (Fin n) R))
              (coeffsIn (Fin n) M ⊓ Submodule.restrictScalars ℤ (degreesLE R (Fin n) B)) (S'.degBound ^ S'.degBound)),
  ∃ (T : ConstructibleSetData R),
    comap MvPolynomial.C '' S.toSet = T.toSet ∧
      ∀ C ∈ T,
        C.n ≤ numBound k (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val d)) (n + 1) ∧
          ∀ (i : Fin C.n), C.g i ∈ M ^ degBound k (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val d)) (n + 1)
Tactic: obtain ⟨T, hT₁, hT₂⟩ := chevalley_polynomialC
      (R := MvPolynomial (Fin n) R)
      (coeffsIn _ M ⊓ (degreesLE _ _ B).restrictScalars ℤ)
      (by simpa [MvPolynomial.coeff_one, apply_ite] using hM)
      S' (fun x hxS j k ↦ by
        simp only [ConstructibleSetData.map,
          RingEquiv.toRingHom_eq_coe, AlgEquiv.toRingEquiv_toRingHom, Finset.mem_image,
          BasicConstructibleSetData.map, RingHom.coe_coe, S', e] at hxS
        obtain ⟨C, hxS, rfl⟩ := hxS
        simp only [comp_apply, Submodule.mem_inf, mem_coeffsIn, Submodule.restrictScalars_mem,
          mem_degreesLE]
        constructor
        · intro d
          simp only [finSuccEquiv_coeff_coeff]
          exact (hS _ hxS _).1 _
        · simp only [B]
          replace hS := (hS _ hxS j).2
          simp only [Submodule.coe_restrictScalars, SetLike.mem_coe, mem_degreesLE,
            Multiset.le_iff_count, Finsupp.count_toMultiset, Finsupp.comapDomain_apply,
            Multiset.toFinsupp_apply, ← degreeOf_def] at hS ⊢
          intro a
          exact (degreeOf_coeff_finSuccEquiv (C.g j) a k).trans (hS _))