ABSTRACT
Author to supply.
ACKNOWLEDGEMENTS
The author would like to thank three anonymous reviewers plus Tony Heron, Bona Muzaka and, especially, Ben Richardson for their helpful comments on earlier drafts of this article. The paper is a revised version of a presentation given at ISA 2010 in New Orleans and many of the insights of the other members of a very stimulating panel (Morten Boas, Andy Cooper, Tim Shaw and Baldur Thorhallsson) have also shaped my thinking; I would like to thank them too.