1. Faculty of Physics, University of Warsaw, Poland
2. Faculty of Mathematics, Ruhr-Universität Bochum, Germany
3. CAMGSD, Departamento de Matemática, Instituto Superior Técnico, Lisboa, Portugal; and Mathematical Institute SANU, Beograd, Serbia
4. Faculty of Physics, University of Warsaw, Poland; and Walter Burke Institute for Theoretical Physics, California Institute of Technology, Pasadena, Cal., U.S.A.