Division of Mathematical Biology, The National Institute for Medical Research The Ridgeway, Mill Hill, London NW7 1AA, UK. kxlin@nimr.mrc.ac.uk|Division of Mathematical Biology, The National Institute for Medical Research The Ridgeway, Mill Hill, London NW7 1AA, UK. kxlin@nimr.mrc.ac.uk|Division of Mathematical Biology, The National Institute for Medical Research The Ridgeway, Mill Hill, London NW7 1AA, UK. kxlin@nimr.mrc.ac.uk|Division of Mathematical Biology, The National Institute for Medical Research The Ridgeway, Mill Hill, London NW7 1AA, UK. kxlin@nimr.mrc.ac.uk