<x xmlns:xi="http://www.w3.org/2001/XInclude">
    <a>text</a>
    <xi:include xpointer="xpointer(//b)"/>
    <xi:include href="404.xml">
        <xi:fallback>
            <b>
                <xi:include xpointer="xpointer(//a)"/>
            </b>
        </xi:fallback>
    </xi:include>
    <xi:include xpointer="xpointer(//b)"/>
</x>
