XPath is not stored within a particular type of document. Instead, XPath expressions are used primarily in XSLT (a transformation language used with XML), but can be used elsewhere as well. (XSLT is covered in more detail in Chapter 6.) In the case of APIs such as 4XPath, expressions can be used against a DOM to return results programmatically in your Python programs. Microsoft’s MSXML3.0 (covered in Appendix E) processes XPath expressions as well.