OPML 2.0, XOXO and XBEL sitting in a tree....

I've been playing around with opml in php, to create a list of links, categorised.
OPML 2.0 is fun, but it seems to have a few fundamental flaws.

The strong point of OPML is that it's easy to understand, easy to extend and easy to build lists with.
But then, working with OPML i find it has some serious issues.

First the schema, there is no official schema. The OPML 2.0 schema i'm using at the moment has some errors in it. Still it is a nice peace of work i can use/reuse to build upon, i found it very helpful.

What troubles me most though is the seemingly lack of development of OPML 2.0 draft.

Another thing that is bothering me is the outlines classification scheme.
If you need to classify an element, and restrict that type to certain conditions, then is it not really another element?

Take for example the outline type link, or include, the outline of this type must have an attribute url.
Why not just create an element link or inlcude? This would give better meaning/structure to the opml file.

Then these is the behavioral attributes, isComment, isBoolean, again markup-smell. outline type comment would be better served with it's own element. I'm not even sure what to think of isBoolean, or worse, isBreakpoint. I think the only thinh worse then isBoolean or BreakPoint is KML's TODO element.

Why? Well it has to do with the difference between a declarative language and an imperative language.

Also, adding new attributes to an OPML2.0 file needs to be done in it's own name space; which is good.
Alternatives to opml, XOXO and OML, seem to miss the point. Using xoxo i could use normal xhtml to create lists, but i would still need to extend it with my own name space.

Using OML, well, i'm not sure if that's possible. I think it's dead.

Then there is XML Bookmark Exchange Language (XBEL) which seems better equiped for exchangin links, but i'm not sure if it's flexible enough for me. So i'm thinking of combining xbel and opml into my own little language.

ATOM is the way to go, much cleaner and more semantic than OPML or any of the other alternatives.

But it's not "schema (xsd)" friendly.
And it's for simple lists only. I would like to build and tree enabled list format for more than just links.

Update:
And so limala is born