I am very grateful to many colleagues in Australia and overseas for encouraging
the thinking reported in this paper and commenting on a draft
typescript. Several of the points made above came from this assistance. I
particularly thank Mr J. Dickenson, Mr J. R. McAlpine, Dr J. S. Russell,
and Dr P. von Carlowitz in this respect. Dr T. H. Booth and Mr M. A. Foale
also gave valuable help.