-- Lean 4 proof step from Mathlib.LinearAlgebra.RootSystem.GeckConstruction.Relations (goal)
Goal:
ι : Type u_1
R : Type u_2
M : Type u_3
N : Type u_4
inst : Finite ι
inst_1 : CommRing R
inst_2 : IsDomain R
inst_3 : CharZero R
inst_4 : AddCommGroup M
inst_5 : Module R M
inst_6 : AddCommGroup N
inst_7 : Module R N
P : RootPairing ι R M N
inst_8 : P.IsCrystallographic
b : P.Base
inst_9 : Fintype ι
i j : ↥b.support
hij : i ≠ j
inst_10 : P.IsReduced
inst_11 : P.IsIrreducible
hij' : (↑i : ι) ≠ (↑j : ι)
this : InvolutiveNeg ι := P.indexNeg
a : ∀ (k l : ↥b.support), ⁅e i, f j⁆ (Sum.inl k) (Sum.inl l) = 0 (Sum.inl k) (Sum.inl l)
⊢ ⁅e i, f j⁆ = 0
have a₁ : ∀ (k : ↥b.support) (l : ι), ⁅e i, f j⁆ (Sum.inl k) (Sum.inr l) = 0 (Sum.inl k) (Sum.inr l)
Tactic: classical
  ext (k | k) (l | l)
  · rw [ne_comm] at hij
    simp_all [-indexNeg_neg, e, f]
  · exact lie_e_f_ne_aux₀ k l
  · have aux₁ : P.root k ≠ P.root i - P.root j :=
      fun contra ↦ b.sub_notMem_range_root i.property j.property ⟨k, contra⟩
    simp [e, f, ← sub_eq_add_neg, if_neg aux₁]
  · /- Geck Case 1 (covered by the auxiliary lemmas above). -/
    rcases eq_or_ne l j with rfl | h₃
    · rw [← ⁅e i, f j⁆.transpose_apply, lie_e_f_ne_aux₁ hij, Pi.zero_apply, Matrix.zero_apply]
    rcases eq_or_ne l (-i) with rfl | h₄
    · rw [← ⁅e i, f j⁆.transpose_apply, lie_e_f_ne_aux₂ hij, Pi.zero_apply, Matrix.zero_apply]
    /- Geck Case 2.
    It's all just definition unfolding and case analysis: the only real content is the external
    lemma `chainBotCoeff_mul_chainTopCoeff`. -/
    suffices
      (∑ x, if P.root x = P.root l - P.root j ∧ P.root k = P.root i + P.root x then
          ((P.chainBotCoeff i x : R) + 1) * (P.chainTopCoeff j l + 1) else 0) =
      (∑ x, if P.root x = P.root i + P.root l ∧ P.root k = P.root x - P.root j then
          ((P.chainTopCoeff j x : R) + 1) * (P.chainBotCoeff i l + 1) else 0) by
      have h₁ : ∀ x ∈ Finset.univ, ¬ ((x = i ∧ l = -i) ∧ k = -j) := by
        rintro - - ⟨⟨-, contra⟩, -⟩; contradiction
      have h₂ : ∀ x ∈ Finset.univ, ¬ ((x = j ∧ l = j) ∧ k = i) := by
        rintro - - ⟨⟨-, contra⟩, -⟩; contradiction
      rw [← sub_eq_zero] at this
      simpa [e, f, ← ite_and, Finset.sum_ite_of_false h₁, Finset.sum_ite_of_false h₂, -indexNeg_neg,
        -Finset.univ_eq_attach]
    by_cases h₅ : P.root l + P.root i - P.root j ∈ range P.root; swap
    · have aux₃ : ∀ x ∈ Finset.univ,
          ¬ (P.root x = P.root i + P.root l ∧ P.root k = P.root x - P.root j) := by
        rintro x - ⟨hx, hx'⟩; exact h₅ ⟨k, by rw [hx', hx]; abel⟩
      have aux₄ : ∀ x ∈ Finset.univ,
          ¬ (P.root x = P.root l - P.root j ∧ P.root k = P.root i + P.root x) := by
        rintro x - ⟨hx, hx'⟩; exact h₅ ⟨k, by rw [hx', hx]; abel⟩
      simp [Finset.sum_ite_of_false aux₃, Finset.sum_ite_of_false aux₄]
    by_cases h₆ : P.root l + P.root i ∈ range P.root; swap
    · have h₇ : P.root l - P.root j ∉ range P.root := by
        rwa [b.root_sub_mem_iff_root_add_mem i j l hij' i.property j.property h₃ _ h₅]
        simpa
      have aux₃ : ∀ x ∈ Finset.univ,
          ¬ (P.root x = P.root i + P.root l ∧ P.root k = P.root x - P.root j) := by
        rintro x - ⟨hx, -⟩; exact h₆ ⟨x, by rw [hx]; abel⟩
      have aux₄ : ∀ x ∈ Finset.univ,
          ¬ (P.root x = P.root l - P.root j ∧ P.root k = P.root i + P.root x) := by
        rintro x - ⟨hx, hx'⟩; exact h₇ ⟨x, hx⟩
      simp [Finset.sum_ite_of_false aux₃, Finset.sum_ite_of_false aux₄]
    obtain ⟨m, hm : P.root m = P.root l - P.root j⟩ :=
      b.root_sub_root_mem_of_mem_of_mem i j l hij' i.property j.property h₅ h₃ h₆
    obtain ⟨l', hl'⟩ := h₆
    by_cases hk : P.root k = P.root l + P.root i - P.root j; swap
    · grind
    have aux₃ (x) (hx : x ≠ m) :
      ¬ (P.root x = P.root l - P.root j ∧ P.root k = P.root i + P.root x) := by
        grind [EmbeddingLike.apply_eq_iff_eq]
    have aux₄ (x) (hx : x ≠ l') :
      ¬ (P.root x = P.root i + P.root l ∧ P.root k = P.root x - P.root j) := by
        grind [EmbeddingLike.apply_eq_iff_eq]
    rw [Finset.sum_eq_single_of_mem m (Finset.mem_univ _) (by rintro x - h; rw [if_neg (aux₃ _ h)]),
      Finset.sum_eq_single_of_mem l' (Finset.mem_univ _) (by rintro x - h; rw [if_neg (aux₄ _ h)]),
      if_pos (⟨hm, by rw [hm, hk]; abel⟩), if_pos ⟨by rw [hl', add_comm], by rw [hl', hk]⟩]
    have := chainBotCoeff_mul_chainTopCoeff i.property j.property hij' hl'.symm hm.symm h₅
    norm_cast