Jonathan Stokes says you can't have multiple children and XML is not relational.  Both very wrong statements.  You can have only one root element.  So in the example <ARTICLES> is the only element that can't be repeated.  You can have as many <ARTICLE> children inside of the root as you want.  The XPATH statements in the article include examples of relational selections.

Admittedly the DOM model used by Explorer has a relational setup that sucks, but it does have one.  I sure wouldn't use it to relate table structures, but it can be done.  (Whether you get an answer this year is another question.)  There are other DOM models that work quite well at doing relationships.