He can be reached at spring@imap.pitt.edu. Copyright (c) 2002 by Michael B. Spring. This document may be reproduced in whole or in part for noncommercial, educational, or scientific purposes, provided that the preceding copyright statement and source are clearly acknowledged. All other rights are reserved. For permission to reproduce or adapt this document or any part of it for commercial distribution, address requests to the author at spring@imap.pitt.edu.