I've put a simple presentation at http://www.cs.auckland.ac.nz/~brian/layers.pdf It's what I would have wanted to present at the RRG meeting, had I been able to be in Vancouver. Brian -- to unsubscribe send a message to rrg-request@psg.com with the word 'unsubscribe' in a single line as the message text body. archive: <http://psg.com/lists/rrg/> & ftp://psg.com/pub/lists/rrg