1. We will use the customary convention Q2 = -q2, where q is the photon momentum four-vector.
2. C. Berger, these proceedings.
3. S.J. Brodsky, SLAC-PUB-2747 (1981);
4. S.J. Brodsky, Proc. 4th Int. Coll. on Photon-Photon Interactions, Paris (1981).
5. G. Altarelli, G. Parisi, Nuc. Phys. B126, 298 (1977).