Acknowledgements
I am grateful to Eduardo Morales, Becky Bixby, Pat Kociolek and an anonymous reviewer for their valuable comments and suggestions on various versions of this paper. In some cases I have not followed their suggestions. I hope they appreciate the reasons for my actions. The late Frank Round and I discussed the contents of this paper at various times during its long gestation. I am not sure he would entirely agree with the final contents but his many suggestions have undoubtedly influenced it.