  Semi-inclusive deep inelastic scattering of polarized leptons off hadrons
enables one to measure the antisymmetric part of the hadron tensor. For
unpolarized hadrons this piece is odd under time reversal. In deep inelastic
scattering it shows up as a $\langle \sin \phi \rangle$ asymmetry for the
produced hadrons. This asymmetry can be expressed as the product of a
twist-three "hadron $\rightarrow$ quark" distribution function and a time
reversal odd twist-two "quark $\rightarrow$ hadron" fragmentation function.
This fragmentation function can only be measured for nonzero transverse momenta
of the produced hadron.
