1. DEPARTMENT OF MATHEMATICS; NATIONAL AND KAPODISTRIAN UNIVERSITY OF ATHENS, PANEPISTIMIOUPOLIS, GR-15784 ATHENS, GREECE AND DEPARTMENT OF INFORMATICS, UNIVERSITY OF BERGEN; N-5020 BERGEN NORWAY
2. DEPARTMENT OF MATHEMATICS, COMPUTER SCIENCE, AND MECHANICS; UNIWERSYTET WARSZAWSKI; WARSAW POLAND
3. DEPARTMENT OF MATHEMATICS; NATIONAL AND KAPODISTRIAN UNIVERSITY OF ATHENS, PANEPISTIMIOUPOLIS, GR-15784 ATHENS, GREECE AND ALGCO PROJECT TEAM; CNRS LIRMM FRANCE