Module: Mathlib.Algebra.Lie.Weights.IsSimple

Goal state before:
pp: K : Type u_1
L : Type u_2
inst✝⁷ : Field K
inst✝⁶ : CharZero K
inst✝⁵ : LieRing L
inst✝⁴ : LieAlgebra K L
inst✝³ : FiniteDimensional K L
inst✝² : IsKilling K L
H : LieSubalgebra K L
inst✝¹ : H.IsCartanSubalgebra
inst✝ : IsTriangularizable K (↥H) L
q : Submodule K (Dual K ↥H)
hq : ∀ (i : ↥LieSubalgebra.root), q ∈ End.invtSubmodule ↑((rootSystem H).reflection i)
χ : Weight K (↥H) L
x_χ m_α : L
hx_χ : x_χ ∈ genWeightSpace L ⇑χ
α : Weight K (↥H) L
hαq : Weight.toLinear K (↥H) L α ∈ q
hα₀ : α.IsNonZero
w_plus : Weight.toLinear K (↥H) L χ + Weight.toLinear K (↥H) L α ≠ 0
w_minus : Weight.toLinear K (↥H) L χ - Weight.toLinear K (↥H) L α ≠ 0
w_chi : Weight.toLinear K (↥H) L χ ≠ 0
m_pos m_neg : L
y : ↥H
hy : y ∈ corootSpace ⇑α
h_bracket_sum : ⁅x_χ, m_α⁆ = ⁅x_χ, m_pos⁆ + ⁅x_χ, m_neg⁆ + ⁅x_χ, ↑y⁆
h_pos_containment : ⁅x_χ, m_pos⁆ ∈ genWeightSpace L (⇑χ + ⇑α)
h_neg_containment : ⁅x_χ, m_neg⁆ ∈ genWeightSpace L (⇑χ - ⇑α)
h_chi_not_in_q : Weight.toLinear K (↥H) L χ ∉ q
S : RootPairing (↥LieSubalgebra.root) K (Dual K ↥H) ↥H := rootSystem H
exists_root_index : ∀ (γ : Weight K (↥H) L), γ.IsNonZero → ∃ i, S.root i = Weight.toLinear K (↥H) L γ
⊢ ⁅x_χ, m_α⁆ ∈ ⨆ α, sl2SubmoduleOfRoot ⋯

Tactic:
have h_plus_bot : genWeightSpace L (χ.toLinear + α.toLinear) = ⊥ := by
    by_contra h_plus_ne_bot
    let γ : Weight K H L := ⟨χ.toLinear + α.toLinear, h_plus_ne_bot⟩
    have hγ_nonzero : γ.IsNonZero := Weight.coe_toLinear_ne_zero_iff.mp w_plus
    obtain ⟨i, hi⟩ := exists_root_index χ (Weight.coe_toLinear_ne_zero_iff.mp w_chi)
    obtain ⟨j, hj⟩ := exists_root_index α hα₀
    have h_sum_in_range : S.root i + S.root j ∈ Set.range S.root := by
      rw [hi, hj]
      exact ⟨⟨γ, by simpa [LieSubalgebra.root]⟩, rfl⟩
    have h_equiv := RootPairing.root_mem_submodule_iff_of_add_mem_invtSubmodule
      ⟨q, by rw [RootPairing.mem_invtRootSubmodule_iff]; exact hq⟩ h_sum_in_range
    rw [hi] at h_equiv
    exact h_chi_not_in_q (h_equiv.mpr (by rw [hj]; exact hαq))

Goal state after:
K : Type u_1
L : Type u_2
inst✝⁷ : Field K
inst✝⁶ : CharZero K
inst✝⁵ : LieRing L
inst✝⁴ : LieAlgebra K L
inst✝³ : FiniteDimensional K L
inst✝² : IsKilling K L
H : LieSubalgebra K L
inst✝¹ : H.IsCartanSubalgebra
inst✝ : IsTriangularizable K (↥H) L
q : Submodule K (Dual K ↥H)
hq : ∀ (i : ↥LieSubalgebra.root), q ∈ End.invtSubmodule ↑((rootSystem H).reflection i)
χ : Weight K (↥H) L
x_χ m_α : L
hx_χ : x_χ ∈ genWeightSpace L ⇑χ
α : Weight K (↥H) L
hαq : Weight.toLinear K (↥H) L α ∈ q
hα₀ : α.IsNonZero
w_plus : Weight.toLinear K (↥H) L χ + Weight.toLinear K (↥H) L α ≠ 0
w_minus : Weight.toLinear K (↥H) L χ - Weight.toLinear K (↥H) L α ≠ 0
w_chi : Weight.toLinear K (↥H) L χ ≠ 0
m_pos m_neg : L
y : ↥H
hy : y ∈ corootSpace ⇑α
h_bracket_sum : ⁅x_χ, m_α⁆ = ⁅x_χ, m_pos⁆ + ⁅x_χ, m_neg⁆ + ⁅x_χ, ↑y⁆
h_pos_containment : ⁅x_χ, m_pos⁆ ∈ genWeightSpace L (⇑χ + ⇑α)
h_neg_containment : ⁅x_χ, m_neg⁆ ∈ genWeightSpace L (⇑χ - ⇑α)
h_chi_not_in_q : Weight.toLinear K (↥H) L χ ∉ q
S : RootPairing (↥LieSubalgebra.root) K (Dual K ↥H) ↥H := rootSystem H
exists_root_index : ∀ (γ : Weight K (↥H) L), γ.IsNonZero → ∃ i, S.root i = Weight.toLinear K (↥H) L γ
h_plus_bot : genWeightSpace L ⇑(Weight.toLinear K (↥H) L χ + Weight.toLinear K (↥H) L α) = ⊥
⊢ ⁅x_χ, m_α⁆ ∈ ⨆ α, sl2SubmoduleOfRoot ⋯