K. Scharnhorst. (1998)cite arxiv:hep-th/9810221Comment: 7 pages Revtex, 5 LaTeX generated figures, file uses axodraw.sty, epic.sty; talk delivered at the workshop ``Superluminal(?) Velocities'', Cologne, July 7-10, 1998, submitted to the Proceedings to appear as a special issue of the Annalen der Physik (v2: typos in the references corrected, minor stylistic changes).