Many people contributed comments and useful suggestions and, in particular, I would like to thank John Denker, Fernando Pereira and David Lewis at Bell Labs, Rob van Meter and Jeff Pickard at AT&T GIS, Doug Monroe from QUEST Bob Waldtein from LINUS and all the people who answered my questions on the ``hotair'' mailing list. To subscribe to that list, send a message to: att.infosystems.www-request@hotair.att.com with 'subscribe' in the Subject line and the 1st line of the body.