1. Department of Computer Science, Durham University Durham DH1 3LE, United Kingdom; E-mail: m.j.r.bordewich@durham.ac.uk
2. Bioinformatics Institute and the Allan Wilson Centre for Molecular Ecology and Evolution, University of Auckland Auckland, New Zealand; E-mail: a.rodrigo@auckland.ac.nz
3. Biomathematics Research Centre, Department of Mathematics and Statistics, University of Canterbury Christchurch, New Zealand; E-mail: c.semple@math.canterbury.ac.nz