  A new diagram method to derive line shape function in the optical
conductivity formula is introduced and the result obtained applying the method
to an electron-phonon system is compared with that derived using the
projection-reduction method. The result satisfies the population criterion,
which states that the distribution functions for electrons and phonons should
be combined in multiplicative forms, and gives physical intuition to quantum
dynamics of electrons in a solid. This method can be called the "KC diagram"
method because it originates from the proper application of the Kang-Choi
reduction identity and a state-dependent projection operator.
