R. Carretero-González1[#!rcg:email!#],
M.J. Ward2 and R.D. Russell1
1Department of Mathematics & Statistics[#!SFU:web!#],
Simon Fraser University, Burnaby, BC, V5A 1S6, Canada
2Department of Mathematics,
University of British Columbia, Vancouver, BC, V6T 1Z2, Canada
June 23, 2000.