When XML is transformed into something else, in most cases XSLT comes to play. One of the challenges of XSLT is to select just the nodes you are interested in. This task is done by XPath, “a query language for selecting nodes from a XML document.”
However, it can be tedious to create a XPath expression, run the transformation, and check if you got the expected result. After hours of debugging you find out: It’s the wrong XPath expression!
To make it easier: Test your XPath expressions in the internal xmllint shell!