Updating another data structure when a value changes

gistfile1.xml

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  More on this at:
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  Copyright (C) 2011 Orbeon, Inc.

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  See the GNU Lesser General Public License for more details.

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<xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
      xmlns:xforms="http://www.w3.org/2002/xforms"
      xmlns:xxforms="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">
    <xhtml:head>
        <xhtml:title>XForms Hello</xhtml:title>
        <xforms:model>
            <xforms:instance>
                <box-results/>
            </xforms:instance>
            <xforms:instance id="codes">
                <box-select>
                    <item selected="false">
                        <label>Cat</label>
                        <value>cat</value>
                    </item>
                    <item selected="false">
                        <label>Dog</label>
                        <value>dog</value>
                    </item>
                    <item selected="false">
                        <label>Bird</label>
                        <value>bird</value>
                    </item>
                    <item selected="false">
                        <label>Fish</label>
                        <value>fish</value>
                    </item>
                </box-select>
            </xforms:instance>
        </xforms:model>
    </xhtml:head>
    <xhtml:body>
        <fr:box-select ref="." id="box-select-control">
            <xforms:action ev:event="xforms-value-changed">
                <xxforms:variable name="selected-values" select="tokenize(., '\s+')"/>
                <xforms:setvalue xxforms:iterate="instance('codes')/item" ref="@selected">false</xforms:setvalue>
                <xforms:setvalue xxforms:iterate="$selected-values"
                    ref="for $v in . return instance('codes')/item
                         [value = $v]/@selected">true</xforms:setvalue>
            </xforms:action>
            <xforms:itemset nodeset="instance('codes')/item">
                <xforms:label ref="label"/>
                <xforms:value ref="value"/>
            </xforms:itemset>
        </fr:box-select>
    </xhtml:body>
</xhtml:html>