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