  At high energies, the relevant degrees of freedom are Wilson lines - infinite
gauge links ordered along straight lines collinear to the velocities of
colliding particles. The effective action for these Wilson lines is determined
by the scattering of QCD shock waves. I develop the symmetric expansion of the
effective action in powers of strength of one of the shock waves and calculate
the leading term of the series. The corresponding first-order effective action,
symmetric with respect to projectile and target, includes both up and down fan
diagrams and pomeron loops.
