Field shown depending on value of other field

relevance.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>
                <instance>
                    <marital-status>single</marital-status>
                    <spouse/>
                </instance>
            </xf:instance>
            <xf:bind ref="/instance/spouse" relevant="/instance/marital-status = 'married'"/>
        </xf:model>
    </xh:head>
    <xh:body>
        <xf:select1 ref="marital-status" appearance="full">
            <xf:label>Marital status</xf:label>
            <xf:item>
                <xf:label>Single</xf:label>
                <xf:value>single</xf:value>
            </xf:item>
            <xf:item>
                <xf:label>Married</xf:label>
                <xf:value>married</xf:value>
            </xf:item>
        </xf:select1>
        <xf:input ref="spouse">
            <xf:label>Spouse</xf:label>
        </xf:input>
    </xh:body>
</xh:html>