1. Mathematics Research Section, School of Mathematical Sciences, The Australian National University, Canberra, ACT 2601, Australia
2. Department of Theoretical Physics, Research School of Physical Sciences, The Australian National University, Canberra, ACT 2601, Australia
3. Institute for High Energy Physics, Serpukhov, Moscow Region, USSR
4. Department of Physics, Oklahoma State University, Stillwater, Ok. 74078–0444, USA