∀ {K : Type u_1} {L : Type u_2} [inst : Field K] [inst_1 : CharZero K] [inst_2 : LieRing L] [inst_3 : LieAlgebra K L]
  [inst_4 : LieAlgebra.IsKilling K L] [inst_5 : FiniteDimensional K L] {H : LieSubalgebra K L}
  [inst_6 : H.IsCartanSubalgebra] [LieModule.IsTriangularizable K (↥H) L] (α β : LieModule.Weight K (↥H) L),
  α.IsNonZero →
    ∀ {x : L}, x ∈ LieAlgebra.rootSpace H ⇑(LieModule.chainTop (⇑α) β) → ∃ n, n • x = ⁅LieAlgebra.IsKilling.coroot α, x⁆