  By examining the symmetry constraints on the semi-inclusive pseudoscalar
particle production in unpolarized inelastic lepton-hadron scattering, we
present a complete, exact Lorentz decomposition for the corresponding hadronic
tensor. As a result, we find that it contains five independent terms, instead
of the four as have been suggested before. The newly identified one is odd
under the naive time reversal transformation, and the corresponding structure
function is directly related to the single spin asymmetry in the semi-inclusive
pseudoscalar meson production by a polarized lepton beam off an unpolarized
target.
