-- Lean 4 proof step from Mathlib.RingTheory.Spectrum.Prime.ChevalleyComplexity (goal)
Goal:
n : ℕ
P : (R : Type u) → [inst : CommRing R] → InductionObj R n → Prop
hP₁ : ∀ (R : Type u) [inst : CommRing R], P R { val := 0 }
hP₂ :
  ∀ (R : Type u) [inst : CommRing R] (e : InductionObj R n) (i : Fin n),
    (e.val i).Monic → (∀ (j : Fin n), j ≠ i → e.val j = 0) → P R e
hP₃ :
  ∀ (R : Type u) [inst : CommRing R] (e : InductionObj R n) (i j : Fin n),
    (e.val i).Monic →
      (e.val i).degree ≤ (e.val j).degree →
        i ≠ j →
          P R { val := Function.update (β := fun (a : Fin n) ↦ R[X]) e.val j (e.val j %ₘ e.val i) : InductionObj R n } →
            P R e
hP₄ :
  ∀ (R : Type u) [inst : CommRing R] (c : R) (i : Fin n) (e : InductionObj R n),
    c = (e.val i).leadingCoeff →
      c ≠ 0 →
        P (Away c)
            {
              val :=
                HSMul.hSMul (α := (Away c)[X]) (β := Fin n → (Away c)[X])
                  ((Polynomial.C : (a : Away c) → (Away c)[X]) (IsLocalization.Away.invSelf c))
                  ((⇑(mapRingHom (algebraMap R (Away c))) : (a : R[X]) → (Away c)[X]) ∘ e.val) :
              InductionObj (Away c) n } →
          P (HasQuotient.Quotient R (B := Ideal R) (Ideal.span {c}))
              {
                val :=
                  (⇑(mapRingHom (algebraMap R (HasQuotient.Quotient R (B := Ideal R) (Ideal.span {c})))) :
                      (a : R[X]) → (R ⧸ Ideal.span {c})[X]) ∘
                    e.val } →
            P R e
R : Type u
inst : CommRing R
e : InductionObj R n
set set₁ : P R e
⊢ P R e
have set₂ : let v := e.degree;
∀ (hv : v = e.degree), P R e
Tactic: classical
  set v := e.degree with hv
  clear_value v
  induction v using WellFoundedLT.induction generalizing R with
  | ind v H_IH =>
    by_cases he0 : e = ⟨0⟩
    · exact he0 ▸ hP₁ R
    cases subsingleton_or_nontrivial R
    · convert! hP₁ R; ext; exact Subsingleton.elim _ _
    simp only [InductionObj.ext_iff, funext_iff, Pi.zero_apply, not_forall] at he0
    -- Case I : The `e i ≠ 0` with minimal degree has invertible leading coefficient
    by_cases H : (∃ i, (e.1 i).Monic ∧ ∀ j, e.1 j ≠ 0 → (e.1 i).degree ≤ (e.1 j).degree)
    · obtain ⟨i, hi, i_min⟩ := H
      -- Case I.ii : `e j = 0` for all `j ≠ i`.
      by_cases! H' : ∀ j ≠ i, e.1 j = 0
      -- then `I = Ideal.span {e i}`
      · exact hP₂ R e i hi H'
      -- Case I.i : There is another `e j ≠ 0`
      · obtain ⟨j, hj, hj'⟩ := H'
        replace i_min := i_min j hj'
        -- then we can replace `e j` with `e j %ₘ (C h.unit⁻¹ * e i) `
        -- with `h : IsUnit (e i).leadingCoeff`.
        apply hP₃ R e i j hi i_min (hj.symm) (H_IH _ ?_ _ rfl)
        refine .left _ _ (lt_of_le_of_ne (b := (ofLex v).1) ?_ ?_)
        · intro k
          simp only [comp_apply, update_apply, hv]
          split_ifs with hjk
          · rw [hjk]
            exact (degree_modByMonic_le _ hi).trans i_min
          · exact le_rfl
        · simp only [hv, ne_eq, not_forall, funext_iff,
            comp_apply]
          use j
          simp only [update_self]
          refine ((degree_modByMonic_lt _ hi).trans_le i_min).ne
    -- Case II : The `e i ≠ 0` with minimal degree has non-invertible leading coefficient
    obtain ⟨i, hi, i_min⟩ : ∃ i, e.1 i ≠ 0 ∧ ∀ j, e.1 j ≠ 0 → (e.1 i).degree ≤ (e.1 j).degree := by
      have : ∃ n : ℕ, ∃ i, (e.1 i).degree = n ∧ (e.1 i) ≠ 0 := by
        obtain ⟨i, hi⟩ := he0; exact ⟨(e.1 i).natDegree, i, degree_eq_natDegree hi, hi⟩
      let m := Nat.find this
      obtain ⟨i, hi, hi'⟩ : ∃ i, (e.1 i).degree = m ∧ (e.1 i) ≠ 0 := Nat.find_spec this
      refine ⟨i, hi', fun j hj ↦ ?_⟩
      refine hi.le.trans ?_
      rw [degree_eq_natDegree hj, Nat.cast_le]
      exact Nat.find_min' _ ⟨j, degree_eq_natDegree hj, hj⟩
    -- We replace `R` by `R ⧸ Ideal.span {(e i).leadingCoeff}` where `(e i).degree` is lowered
    -- and `Away (e i).leadingCoeff` where `(e i).leadingCoeff` becomes invertible.
    apply hP₄ _ _ i e rfl (by simpa using hi) (H_IH _ ?_ _ rfl) (H_IH _ ?_ _ rfl)
    · rw [hv, Prod.Lex.lt_iff']
      constructor
      · intro j
        simp only [coe_mapRingHom, InductionObj.ofLex_degree_fst, Pi.smul_apply,
          comp_apply, smul_eq_mul]
        refine ((degree_mul_le _ _).trans (add_le_add degree_C_le degree_map_le)).trans ?_
        simp
      rw [lt_iff_le_not_ge]
      simp only [coe_mapRingHom, funext_iff, InductionObj.ofLex_degree_fst, Pi.smul_apply,
        comp_apply, smul_eq_mul, show (ofLex e.degree).2 from H,
        le_Prop_eq, implies_true, true_implies, true_and]
      simp only [InductionObj.ofLex_degree_snd, Pi.smul_apply, comp_apply, smul_eq_mul,
        ne_eq, not_exists, not_and, not_forall, not_le, not_lt]
      intro h_eq
      refine ⟨i, ?_, ?_⟩
      · rw [Monic.def, ← coeff_natDegree (p := _ * _), natDegree_eq_of_degree_eq (h_eq i),
          Polynomial.coeff_C_mul, Polynomial.coeff_map, coeff_natDegree, mul_comm,
          IsLocalization.Away.mul_invSelf]
      · intro j hj; rw [h_eq, h_eq]; exact i_min j fun H ↦ (by simp [H] at hj)
    · rw [hv]
      refine .left _ _ (lt_of_le_of_ne ?_ ?_)
      · intro j; simpa using degree_map_le
      simp only [coe_mapRingHom, ne_eq]
      intro h_eq
      replace h_eq := congr_fun h_eq i
      simp only [Ideal.Quotient.algebraMap_eq, comp_apply, degree_map_eq_iff,
        Ideal.Quotient.mk_singleton_self, ne_eq, not_true_eq_false, false_or] at h_eq
      exact hi h_eq