  In the framework of a mass formula proposed previously (transforming in a
specific way three free parameters into three masses), a simple empirical sum
rule for three charged-lepton masses is found, predicting m_\tau = 1776.9926
MeV, when the experimental values of m_e and m_\mu are used as an input. The
experimental value to be compared with is m_\tau = 1776.99^{+0.29}_{-0.26} MeV.
This satisfactory sum rule (equivalent to a simple parameter constraint in the
mass formula) is linear in masses and involves integers as its coefficients.
The author believes that such a simple and precise mass sum rule for charged
leptons may help in the process of developing realistic models for mass spectra
of fundamental fermions. In the second part of the paper, another equivalent
parametrization of the charged-lepton mass formula is described, corresponding
to an oscillatory picture of their mass matrix, where two matrices appear
playing the role of annihilation and creation operators in the generation
space.
