1. Department of Biochemistry, The University of Oxford, South Parks Road, Oxford, OX1 3QU, U.K.
2. Universität Potsdam, Institut für Mathematik, Am Neuen Palais 10, D-14469 Potsdam, Germany
3. Fujitsu Laboratories of Europe Ltd. (FLE), Hayes Park Central, Hayes End Road, Hayes, UB4 8FE, U.K.