  A model-independent analysis of collinear three-parton correlation functions
for fragmentation is performed. By investigating their support properties it is
shown, in particular, that the so-called partonic pole matrix elements vanish.
This sheds new light on the understanding of transverse single spin asymmetries
in various hard semi-inclusive reactions. Moreover, it gives additional strong
evidence for the universality of transverse-momentum-dependent fragmentation
functions.
