Computer Science and Mathematics, School of Natural Sciences, Stirling University, Stirling, FK9 4LA, UK. abb@cs.stir.ac.uk.|Department of Computer Science, University of Torino, Torino, Italy.|Laboratoire de Biométrie et Biologie Evolutive, University Claude Bernard, Lyon, France.|Center for Bioinformatics, Saarland University, Saarland, Germany.|Department of Computer Science, University of Pisa, Pisa, Italy.|Institute of Biomedical Technologies, National Research Council, Milan, Italy.|Department of Computer Science, University of Pisa, Pisa, Italy.