Institute of Applied Mathematics, University of British Columbia, Vancouver, BC, Canada. rosalia@cs.ubc.ca <rosalia@cs.ubc.ca>|Institute of Applied Mathematics, University of British Columbia, Vancouver, BC, Canada. rosalia@cs.ubc.ca <rosalia@cs.ubc.ca>|Institute of Applied Mathematics, University of British Columbia, Vancouver, BC, Canada. rosalia@cs.ubc.ca <rosalia@cs.ubc.ca>