Custom Control Settings storing a question identifier in the data

control-settings.xbl

<xbl:xbl xmlns:xh="http://www.w3.org/1999/xhtml"
         xmlns:xf="http://www.w3.org/2002/xforms"
         xmlns:xs="http://www.w3.org/2001/XMLSchema"
         xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
         xmlns:fr="http://orbeon.org/oxf/xml/form-runner"
         xmlns:xbl="http://www.w3.org/ns/xbl"
         xmlns:xxbl="http://orbeon.org/oxf/xml/xbl"
         xmlns:acme="http://www.acme.com/xbl">

    <xbl:binding element="acme|control-settings" id="acme-control-settings">
        <xbl:handlers>
            <xbl:handler event="fb-initialize" phase="target">
                <xf:setvalue
                    ref="instance('i')/@question-identifier"
                    value="event('data-holders')/@question-identifier"/>
            </xbl:handler>
            <xbl:handler event="fb-apply" phase="target">
                <xf:action iterate="event('data-holders')">
                    <xf:delete ref="@question-identifier"/>
                    <xf:insert context="." origin="instance('i')/@question-identifier"/>
                </xf:action>
            </xbl:handler>
        </xbl:handlers>
        <xbl:implementation>
            <xf:model>
                <xf:instance id="i">
                    <_ question-identifier=""/>
                </xf:instance>
            </xf:model>
        </xbl:implementation>
        <xbl:template>
            <xh:div>
                <xf:input ref="instance('i')/@question-identifier">
                    <xf:label>Question identifier</xf:label>
                </xf:input>
            </xh:div>
        </xbl:template>
    </xbl:binding>

</xbl:xbl>