gistfile1.html

<xh:html
        xmlns:xh="http://www.w3.org/1999/xhtml"
        xmlns:xf="http://www.w3.org/2002/xforms"
        xmlns:ev="http://www.w3.org/2001/xml-events"
        xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xh:head>
        <xf:model>
            <xf:instance id="persons">
                <persons>
                    <person>
                        <id>marx</id>
                        <name>Karl Marx</name>
                    </person>
                    <person>
                        <id>engels</id>
                        <name>Friedrich Engels</name>
                        <friend>marx</friend>
                    </person>
                </persons>
            </xf:instance>
        </xf:model>
    </xh:head>
    <xh:body>
        <xf:group ref="instance('persons')/person[id = 'engels']">
            <xf:output ref="instance('persons')/person[id=current()/friend]/name"/>
        </xf:group>
    </xh:body>
</xh:html>