-- 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 (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)
N : ℕ := (k * (1 + Multiset.count 0 d)) ^ (k * (1 + Multiset.count 0 d))
this :
  ∀ (C : BasicConstructibleSetData (MvPolynomial (Fin n) R)) (hCT : C ∈ T) (a : Fin C.n),
    Membership.mem (γ := Submodule ℤ (MvPolynomial (Fin n) R))
      (coeffsIn (Fin n) (M ^ N) ⊓ Submodule.restrictScalars ℤ (degreesLE R (Fin n) (N • B))) (C.g a)
h1M : 1 ≤ M
⊢ ∃ (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 : ∀ (U : ConstructibleSetData R) (hU₁ : comap MvPolynomial.C '' T.toSet = U.toSet)
  (hU₂ :
    ∀ C ∈ U,
      C.n ≤ numBound S'.degBound (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val (N • B))) n ∧
        ∀ (i : Fin C.n),
          C.g i ∈ (M ^ N) ^ degBound S'.degBound (fun (i : ℕ) ↦ 1 + Multiset.count i (Multiset.map Fin.val (N • B))) n),
  ∃ (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 ⟨U, hU₁, hU₂⟩ := IH (M := M ^ N)
    (SetLike.le_def.mp (le_self_pow h1M Nat.pow_self_pos.ne') hM) _ _ T
    (fun C hCT ↦ (hT₂ C hCT).1)
    (fun C hCT k ↦ this C hCT k)