With the recent and rapid advance of the Internet, management of structured documents such as XML documents and their databases has become more and more important. A number of query languages for XML documents have been proposed up to the present. Some of them enable tag-based powerful document structure manipulation. However, their contents processing capability is very limited. Here, the contents processing implies the similarity-based selection, ranking, summary generation, topic extraction, and so on, as well as simple string-based pattern matching. In this paper, we propose an extensible XML query language X²QL, which features inclusion of user-defined foreign functions to process document contents in the context of XML-QL-based document structure manipulation. This feature makes it possible to integrate application-oriented high-level contents processing facilities into querying documents. We also describe an implementation of an X QL query processing systemon top of XSLT processors.