  Generalized parton distributions of the nucleon are accessed via exclusive
leptoproduction of the real photon. While earlier analytical considerations of
phenomenological observables were restricted to twist-three accuracy, i.e.,
taking into account only terms suppressed by a single power of the hard scale,
in the present study we revisit this differential cross section within the
helicity formalism and restore power-suppressed effects stemming from the
process kinematics exactly. We restrict ourselves to the phenomenologically
important case of lepton scattering off a longitudinally polarized nucleon,
where the photon flips its helicity at most by one unit.
