  Nucleon structure functions can be observed in Deep Inelastic Scattering
experiments, but it is an outstanding challenge to confront them with fully
non-perturbative QCD results. For this purpose we investigate the product of
electromagnetic currents (with large photon momenta) between quark states (of
low momenta). By means of an Operator Product Expansion the structure function
can be decomposed into matrix elements of local operators, and Wilson
coefficients. For consistency both have to be computed non-perturbatively. Here
we present precision results for a set of Wilson coefficients. They are
evaluated from propagators for numerous quark momenta on the lattice, where the
use of chiral fermions suppresses undesired operator mixing. This
over-determines the Wilson coefficients, but reliable results can be extracted
by means of a Singular Value Decomposition.
