href attribute identifies the document (or part thereof)
to be included
In the http://www.w3.org/1999/XML/xinclude namespace.
<book xmlns:xinclude="http://www.w3.org/1999/XML/xinclude">
<title>Processing XML with Java</title>
<chapter><xinclude:include href="dom.xml"/></chapter>
<chapter><xinclude:include href="sax.xml"/></chapter>
<chapter><xinclude:include href="jdom.xml"/></chapter>
</book>