$doc.display

This is a two-part question: First, somewhere on xwiki.org, I found a discussion Ludovic had about the design strategy of XWiki, and I seem to recall that he mentioned that the property types in XWiki objects were extensible, which I can well believe. However, I have searched in vain for any mention of how to do this, and it looks as though it requires adding Java classes to the server, and at minimum a database schema change. However, some hope remains because it seems that there may be a way to create aggregate objects from simple types. How can this be done? Second, how do I learn the different display modes and what they mean to the $doc.display method? I note that the "xcode" parameter's value sometimes corresponds to a Velocity script (e.g. "rdf" to rdf.vm, etc.). What I would like to be able to do is to create a custom display mode so that, in conjunction possibly with an aggregate type, I can, for example, create an arbitrarily-deep hierarchical menu structure, and when I invoke a particular xcode or other mode, $doc.display will use my template to decorate the data with xml that will make links that invoke javascript to show or hide their contents. In general, it appears there are lots of very valuable constants (format strings, display modes, special codes, etc.) throughout the system that are not useful to me because I don't know about them. The javadoc APIs hint at them, and some of the wiki docs use them. How can I learn of them, especially those that are simply the result of configurations that I can easily extend? This is a wonderful product, and developing with it is fabulously easy - once I figure out stuff, which is excruciatingly difficult. I am sure that you are working furiously on the project, but I wonder how much more effectively you could work if the APIs were more effectively documented? Then lots more work could get done, because others would be able to work much more effectively, and have plenty of time left over to contribute our stuff back. That's how it would work with me, at least... brain[sic]