  In a consistently formulated pQCD framework incorporating non-zero mass heavy
quark partons, there is still the freedom to define parton distributions
obeying either mass-independent or mass-dependent evolution equations, contrary
to statements made in a recent paper by MRRS. With properly matched hard
cross-sections, different choices merely correspond to different factorization
schemes, and they yield the same physical cross-sections. We demonstrate this
principle in a concrete order \alpha_s calculation of the DIS charm structure
function. We also examine the proper matching between parton definitions and
subtractions in the hard cross-section near threshold where the calculation is
particularly sensitive to mass effects of the heavy quark. The results obtained
from the general-mass formalism are quite stable against different choices of
scale and exhibit a smooth transition in the threshold region (using either
mass-independent or mass-dependent evolution), in contrast to results of
another recently proposed scheme.
