Editors-in-Chief
E. Dudas, M. Elsing, F. Forti, J. Monroe, D.J. Schwarz and G. Zanderighi

The author would like to thank two anonymous referees for pointing out several shortcomings in a previous version of this paper and for suggestions to improve its clarity.

J. H. Field

ISSN: 1434-6052 (Electronic Edition)

© Società Italiana di Fisica and
Springer-Verlag