feat(algebra/lie/cartan_subalgebra): add lemma `lie_subalgebra.exists_nested_lie_ideal_of_le_normalizer` (#11820)
feat(algebra/lie/cartan_subalgebra): add lemma `lie_subalgebra.exists_nested_lie_ideal_of_le_normalizer` (#11820)






leanprover-community/mathlib,leanprover-community/mathlib