Jelly is not currently used in Cocoon. The idea is interesting, but it also means a rewrite of a large part of the sitemap engine, and doesn't give an immediate answer to the mixing between components and container outlined by Berin.
To further my line of thought... The current object model or the sitemap would be converted from pseudo-objects to a more bean-like structure. Jelly would just replace the XXXBuilder objects, and also take care of managing what elements could be nested inside of others, etc.
Note that I don't state this to protect the current treeprocessor, but to minimize the work required to move to Fortress. Now I recognize I don't know Jelly and thus how hard this would be.
Now that I brought this up, I just might have to see how easy it will be :)
-pete
