  We argue that weighted azimuthal single spin asymmetries in back-to-back jet
or pion production in polarized proton-proton scattering can be written as
convolutions of universal distribution and fragmentation functions with gluonic
pole cross sections as hard functions. Gluonic pole cross sections are
gauge-invariant weighted sums of Feynman diagrams. The weight factors are a
direct consequence of the (diagram-dependence of) gauge links. The best known
consequence of the gauge links is the generation of the Sivers effect that is a
source for single-spin asymmetries. Moreover, due to the dependence of the
gauge links on the color-flow of the hard diagram the Sivers effect in SIDIS
enters with opposite sign as it does in Drell-Yan scattering. The weight
factors in the gluonic pole cross sections are the appropriate generalizations
to more complicated processes of this relative sign difference. Furthermore, it
is argued that the gluon-Sivers effect appears in twofold.
