1. Department of Mathematics, University of Florida, Gainesville, FL 32611
2. Institut Elie Cartan, Laboratoire de Mathématiques, Université Henri Poincaré Nancy I, B.P. 239, 54506 Vandoeuvre Lès Nancy Cedex, France; Systems Research Institute of the Polish Academy of Sciences, ul. Newelska 6, 01-447 Warszawa, Poland