Author(s): P. Mayr
-- hep-th/0010223 was replaced with a revised version
on Thu, 2 Nov 2000 22:29:36 GMT (49kb) --
Paper: hep-th/0010223
Comments: 46 pages, harvmac (b), 3 figures;v2: a few relevant additions and
corrections
Report-no: CERN-TH/2000-315
URL: http://arXiv.org/abs/hep-th/0010223