1. McGill University, Dept. of Mech. Eng. & Centre for Intelligent Machines, 817 r. Sherbrooke O., Montre´al, Que´bec, Canada
2. Universita¨tsdozent fu¨r Geometrie an der Montanuniversita¨t Leoben, Franz-Josef-Straße 18, A-8700 Leoben, O¨sterreich and McGill University, Centre for Intelligent Machines, 817 r. Sherbrooke O., Montre´al, Que´bec, Canada
3. McGill University, Dept. of Mech. Eng. & Centre for Intelligent Machines, 817 r. Sherbrooke O., Montre´al, Que´bec, Canada and Institut fu¨r Geometrie, TU Graz, Kopernikusgasse 24, A-8010 Graz, O¨sterreich