laemmerzahl(at)uni-duesseldorf.de
Received 13 July 2001, revised 31 October 2001, accepted 2 November 2001 by F. W. Hehl
Faculty of Physics and Center for Theoretical Physics, University of Craiova, 13 A. I. Cuza Str., Craiova RO-1100, Romania; bizdadea(at)central.ucv.ro
Received 13 August 2001, accepted 25 September 2001 by K. Fredenhagen
II. Physikalisches Institut, Universtät zu Kön, 50923 Kön, Germany; g.nimtz(at)uni-koeln.de, ah(at)ph2.uni-koeln.de
Received 6 February 2001, revised 14 October 2001, accepted 15 October by F. W. Hehl