  Quark-quark double scattering in eA DIS and its contribution to quark and
anti-quark fragmentation functions are investigated with the generalized
factorization of the relevant twist-four processes in pQCD. It is shown that
the resulting modifications to quark and anti-quark fragmentation functions are
different. While the numerical size of these effects cannot be determined from
pQCD, the structure of our result leads to a number of qualitative predictions
for the relative size of the effect for different hadrons. These qualitative
predictions agree with the multiplicity ratios for positive and negative
hadrons as observed by HERMES.
