Alex highlights how librarians misuse xml in relation to marc. There's a deeper theme that applies to xml schemas outside the library world as well that I've struggled with. Codification with angle brackets doesn't get you much apart from a feature checkbox in the product marketing. Semantics in a codified manner are a very powerful thing that reduces development time and complexity. Schema's and xml developed like this fail the fundamental test when adopting a new technology, does it make things easier and simpler?