into two categories: the long-distance and the short-distance contributions due to the fact that every “on-shell” propagator can have a pole contribution as well as a contact contribution [@Qiu]. For example, a quark propagator of momentum $k$ can always be written as $$\frac{i\gamma\cdot k}{k^2} 
= \frac{i\gamma\cdot \hat{k}}{k^2}
+ \frac{i\gamma\cdot n}{2k\cdot n}\ ,
\label{DF0}$$ where $\hat{k}^2 = 0$ and $n^\mu$ is any auxiliary vector with $k\cdot
n \neq 0$. The first term in the right-hand-side of Eq. (\[DF0\]) corresponds the pole contribution, while the second term is the contact contribution [@Qiu]. Attaching one gluon to the initial quark line introduces a quark propagator, and this propagator will have both the pole and contact contributions. The pole contribution is long-distance in nature, representing the interactions between the quark and the gluon long before the hard collision between the quark and the virtual photon. The pole contribution is partially responsible for the $A$-dependence of the leading-twist parton distributions in a nucleus [@CQR], and therefore, is one of the sources for the weak $A$-dependence appeared in the $a$ term in Eq. (\[BA13\]).

Because of the nature of the contact term, its contribution is localized in space, and therefore, it does not contribute to the 