Coolest. Thing. Ever. I hope this RELAX NG thing is going to be really big.

It's great to see how easy it is to provide modular document types. I wish docbook 5 would use that technique and organize its markup into such modules, such that things like modeling-like markup (function, class, classsynpsis, etc.) would all end up in their respecitive 'profile', so it would be easier to define a 'docbook-core' and a set of 'profiles', which users could select individually. That would facilitate discussing modifications and additions to individual profiles.

I think that removing unnecessary elements is more usable way of doing things. Building the schema from an individual modules means that you must be very careful about content models. You can do this in RNG using combine construct, but many elements would fall into several modules (especially in case of inlines).

To make customization easier it is possible to group notAllowed elements together and create small grammars that can be included to switch off whole set of related elements.

Is this the answer to the 'simple' docbook that came up on the apps list some time back?

I guess the use case is people wanting to generate docbook to X stylesheets, beaten by the recursion and complexity of the current content models?

Another benefit of moving to Relax NG Norm?

I've been handling my customization layers this way for several months. Since I use oXygen, I can use the RelaxNG validation right away. I can also transform to a DTD very easily. XML Hack #78 describes this process in detail. And I agree with the earlier comment: Best thing ever!

very cool. What tool would you use to edit such a micro vocabulary?

I'd use Emacs, as ever. But any RELAX NG aware editor should work, and there are more of them all the time.