1. Department of Mathematical and Life Sciences, Hiroshima University, Higashi‐Hiroshima 739‐8526, Japan;
2. Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi 332‐0012, Japan;
3. Institute of Life Sciences, Ajinomoto Co., Inc., Kawasaki 210‐8681, Japan