Using xxforms-value-changed with a test on the node changed

form.xhtml

<xh:html xmlns:xh="http://www.w3.org/1999/xhtml"
      xmlns:xf="http://www.w3.org/2002/xforms"
      xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
      xmlns:ev="http://www.w3.org/2001/xml-events"
      xmlns:xs="http://www.w3.org/2001/XMLSchema"
      xmlns:fr="http://orbeon.org/oxf/xml/form-runner">
    <xh:head>
        <xf:model>
            <xf:instance id="my-instance">
                <instance>
                    <a/>
                    <b/>
                </instance>
            </xf:instance>
            <xf:message event="xxforms-value-changed"
                        observer="my-instance"
                        if="event('node') = instance()/a"
                        value="'Changed'"/>
        </xf:model>
    </xh:head>
    <xh:body>
        <xf:trigger>
            <xf:label>Set value</xf:label>
            <xf:setvalue event="DOMActivate" ref="a">42</xf:setvalue>
        </xf:trigger>
    </xh:body>
</xh:html>