  I follow a lesson learnt from Heisenberg's matrix Quantum Mechanics to study
the property of lepton flavor mixing and CP violation. I show that the
commutator of lepton mass matrices defined in vacuum keeps invariant under
terrestrial matter effects within the four-neutrino mixing scheme. A set of
model-independent sum rules for neutrino masses are obtained, and they can be
generalized to hold for an arbitrary number of neutrino families. Useful sum
rules for the rephasing-invariant measures of leptonic CP violation have also
been found. Finally I present a generic formula of T-violating asymmetries
applicable to the future long-baseline neutrino oscillation experiments.
