Prototype for XBL component with binding to optional element

gistfile1.xml

<xbl:xbl xmlns:xhtml="http://www.w3.org/1999/xhtml"
         xmlns:xforms="http://www.w3.org/2002/xforms"
         xmlns:ev="http://www.w3.org/2001/xml-events"
         xmlns:xxforms="http://orbeon.org/oxf/xml/xforms"
         xmlns:fr="http://orbeon.org/oxf/xml/form-runner"
         xmlns:oxf="http://www.orbeon.com/oxf/processors"
         xmlns:xbl="http://www.w3.org/ns/xbl"
         xmlns:xxbl="http://orbeon.org/oxf/xml/xbl">

    <xbl:binding id="fr-optional-element" element="fr|optional-element" xxbl:mode="binding handlers" xxbl:container="span">
        <xbl:template>
            <xforms:group appearance="xxforms:internal">
                <!-- Content such as xforms:input -->
                <xbl:content/>

                <!-- Insert element if missing -->
                <xforms:action ev:event="xforms-enabled" ev:target="#observer">
                    <xforms:var name="ref" xbl:attr="xbl:text=ref"/>
                    <xforms:var name="name" value="tokenize($ref, '/')[last()]"/>
                    <xforms:insert
                        if="not(exists(xxforms:binding('fr-optional-element'))) and $name castable as xs:NCName"
                        context="xxforms:component-context()"
                        ref="*"
                        origin="xxforms:element($name)"/>
                </xforms:action>

            </xforms:group>

        </xbl:template>
    </xbl:binding>
</xbl:xbl>