Each of the papers in this volume has been selected because the authors
are `well-known', and their reputation was a significant factor in this
selection process. But just because someone has done superb work in
the past does not mean that they are doing so now. Indeed, the very
best of us make mistakes (in our view, the only people who make no
mistakes are those who do nothing at all, for fear of making
mistakes). So selecting authors on the basis of reputation is not
enough. Additionally, many of the underlying papers have or will have
appeared elsewhere, in refereed journals. In fact, we solicited
particular papers from these authors, partly to ease the burden for the
authors, partly to allow us to select papers with a good potential for
``activation", and partly because the refereeing issue will be less
urgent for such works: only updates or changes ought to need scrutiny
(though even fatal mistakes will slip past good referees on occasion).