TREX update

[James Clark <jjc@jclark.com>]

I've updated the TREX website

  http://www.thaiopensource.com/trex/

The only change to the language is that I've refined the way it handles
merging included grammars. See

  http://www.thaiopensource.com/trex/tutorial.html#Merging_grammars

for more information.

I've also done a complete implementation of XHTML modularization in
TREX.  See

 http://www.thaiopensource.com/trex/xhtml/

This has been done by hand not automatically, so there are probably
bugs, but it takes full advantage of TREX.  Notably, with TREX there's
no need to create a model module specific to the combination of modules
you are using; you just have to include the modules you need.  I was
also pleasantly surprised to discover that it was possible to implement
exceptions in TREX (used in HTML 4 to, for example, prevent nested
links), even though there's no specific exceptions feature in TREX. I've
added an explanation of this technique to the tutorial:

  http://www.thaiopensource.com/trex/tutorial.html#Concur

The sample implementation has also been updated to match the language
spec and fix some bugs.

James