∀ {K : Type u_1} {L : Type u_2} [inst : Field K] [CharZero K] [inst_2 : LieRing L] [inst_3 : LieAlgebra K L]
  [LieAlgebra.IsKilling K L] [FiniteDimensional K L] {H : LieSubalgebra K L} [inst_6 : H.IsCartanSubalgebra]
  [LieModule.IsTriangularizable K (↥H) L] (α : LieModule.Weight K (↥H) L),
  α.IsNonZero → LieAlgebra.rootSpace H (2 • ⇑α) = ⊥