Author(s): G. Pinter
-- hep-th/9911063 was replaced with a revised version
on Wed, 1 Nov 2000 13:36:33 GMT (28kb) --
Paper: hep-th/9911063
Comments: 32 pages, added reference, corrected typos and changed content
Report-no: DESY 99-168
URL: http://arXiv.org/abs/hep-th/9911063