  We generalize the spectral sum rule preserving density matrix numerical
renormalization group (DM-NRG) method in such a way that it can make use of an
arbitrary number of not necessarily Abelian, local symmetries present in the
quantum impurity system. We illustrate the benefits of using non-Abelian
symmetries by the example of calculations for the T-matrix of the two-channel
Kondo model in the presence of magnetic field, for which conventional NRG
methods produce large errors and/or take a long run-time.
