<a>
    <xi:include href="../ents/something.xml" xmlns:xi="http://www.w3.org/2001/XInclude">
        <xi:fallback>
            <xi:include href="c.xml">
                <xi:fallback>There is no c.xml ... </xi:fallback>
            </xi:include>
        </xi:fallback>
    </xi:include>
</a>