=====================

NLO parton-level cross section
------------------------------

The starting point for the construction of MC@NLO is that of writing the short-distance parton-level cross section according to the subtraction formalism of refs. [@Frixione:1995ms; @Frixione:1997np] (which we shall call FKS subtraction henceforth). The default procedure in FKS is that of treating simultaneously (i.e. in one contribution to the partonic cross section) the two initial-state collinear singularities, due to one given final-state parton being collinear to the initial-state parton coming from the left or from the right. On the other hand, in FKS one can also treat these two singularities independently, by defining two separate contributions to the short-distance cross section, each of which corresponds to one of the initial-state singularities; this procedure is explained in detail in ref. [@Frederix:2009yq]. As we shall discuss in the following, when an NLO computation is matched to  according to the MC@NLO formalism, it turns out to be convenient to adopt the latter strategy (at variance with the case of , where the simultaneous subtraction suffices).

We shall assign the momenta entering the (real-emission) partonic subprocesses as follows: a(p\_1)+b(p\_2)V(k\_1)+c(k\_2), \[Rproc\] with $V$ representing a $W$ or a $Z$ boson, and $a$, $b$, and $c$ being QCD partons; Born-like 