1. Barnett, J. A., Nature New Biology (in the press).
2. Barnett, J. A., Antonie van Leeuwenhoek, 23, 1 (1957).
3. Gower, J. C., in discussion on Marshal, A. W., and Olkin, I., J. Roy. Statist. Soc., B, 30, 438 (1968).
4. Pankhurst, R. J., Computer J., 13, 145 (1970).
5. Pankhurst, R. J., Nature, 227, 1269 (1970).