Bioinformatics Research Centre, Department of Computing Science, University of Glasgow, Glasgow, UK. srogers@dcs.gla.ac.uk|Bioinformatics Research Centre, Department of Computing Science, University of Glasgow, Glasgow, UK. srogers@dcs.gla.ac.uk|Bioinformatics Research Centre, Department of Computing Science, University of Glasgow, Glasgow, UK. srogers@dcs.gla.ac.uk