Acknowledgements

MD thanks Geoff Bicknell, Ralph Sutherland, Pete McGregor, Charlene Heisler, Agris Kalnajs, Mark Whittle and Wil van Breugel for many stimulating conversations and physical insights in the preparation of this work. The comments of the referees were very welcome, and helped to me to clarify a number of aspects of the paper that were clear only to the author. I also wish to thank the Australian Dept. of Industry, Science and Tourism (DIST) for support under a International Science and Technology (IS&T) major grant, which covered travel in the preparation of this work. This research has made use of NASA's (invaluable) Astrophysics Data System Abstract Service.

© Copyright Astronomical Society of Australia 1997