Abstract
The widespread use of the eXtensible Markup Language (XML) in scientific data repositories, digital libraries and on the web, brought about an explosion in the development of XML retrieval systems. These systems exploit the logical structure of documents, which is explicitly represented by the XML markup: instead of whole documents, only components thereof (the so-called XML elements) are retrieved in response to a user query. This means that an XML retrieval system needs not only to find relevant information in the XML documents, but also determine the appropriate level of granularity to return to the user, and this with respect to both content and structural conditions.