XForms: Clearing non-relevant elements upon submission

gistfile1.html

<xh:html
        xmlns:xs="http://www.w3.org/2001/XMLSchema"
        xmlns:xh="http://www.w3.org/1999/xhtml"
        xmlns:xf="http://www.w3.org/2002/xforms"
        xmlns:exf="http://www.exforms.org/exf/1-0"
        xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
        xmlns:ev="http://www.w3.org/2001/xml-events">
    <xh:head>
        <xf:model id="model">
            <xf:instance id="instance">
                <instance relevant="true">
                    <a/>
                    <b/>
                </instance>
            </xf:instance>
            <xf:bind ref="a" relevant="instance()/@relevant = 'true'"/>
            <xf:bind ref="b" relevant="instance()/@relevant = 'false'"/>

            <xf:submission id="save" relevant="false" ref="instance()" action="echo:" method="post" replace="instance">
                <xf:setvalue ev:event="xforms-submit" xxf:iterate="//*[not(exf:relevant(.))]" ref="."/>
            </xf:submission>
        </xf:model>
    </xh:head>
    <xh:body>
        <xf:input ref="a"/>
        <xf:input ref="b"/>

        <xf:trigger>
            <xf:label>Toggle</xf:label>
            <xf:setvalue ev:event="DOMActivate" ref="@relevant" value="if (. = 'true') then 'false' else 'true'"/>
        </xf:trigger>

        <xf:submit submission="save">
            <xf:label>Save</xf:label>
        </xf:submit>

        <fr:xforms-inspector xmlns:fr="http://orbeon.org/oxf/xml/form-runner"/>
    </xh:body>
</xh:html>