I wrote most of this up in response to someone's off-list question and
thought it worth throwing in.  PWS, feel free to move it out of
chapter 2 if it would go better elsewhere ... I didn't want to mess up
numbering and cross-references by attempting to add a whole new
chapter.

Also made some (but probably not all that are needed) edits to bring a
few answers and acknowledgements up to date.  I'm not sure after 28
years that I still deserve special mention for making suggestions, but
left that alone.