Centrum Wiskunde & Informatica (CWI), P,O, Box 94079, 1090 GB, Amsterdam, The Netherlands. l.j.j.v.iersel@gmail.com.|National Institute for Basic Biology, National Institutes of Natural Sciences, Nishigonaka 38, Myodaiji, Okazaki 444-8585, Japan. uchiyama@nibb.ac.jp.