Disabling a specific radio button with XForms

gistfile1.xml

<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>
        <xforms:model>
            <xforms:instance>
                <instance>
                    <add/>
                    <delete/>
                    <add-delete/>
                    <delete-enabled>true</delete-enabled>
                </instance>
            </xforms:instance>
            <xforms:bind ref="add-delete" calculate="string-join((../add, ../delete), ' ')"/>
            <xforms:bind ref="delete" readonly="../delete-enabled = 'false'"/>
        </xforms:model>
    </xhtml:head>
    <xhtml:body>
        <xforms:trigger>
            <xforms:label>Toggle delete enabled</xforms:label>
            <xforms:setvalue ev:event="DOMActivate" ref="delete-enabled" value="if (. = 'true') then 'false' else 'true'"/>
        </xforms:trigger>
        <xforms:output ref="instance()/add-delete"/>
        <xforms:select1 ref="instance()/add" appearance="full">
            <xforms:item>
                <xforms:label>Add</xforms:label>
                <xforms:value>A</xforms:value>
            </xforms:item>
            <xforms:setvalue ev:event="xforms-select" ref="instance()/delete"/>
        </xforms:select1>
        <xforms:select1 ref="instance()/delete" appearance="full">
            <xforms:item>
                <xforms:label>Delete</xforms:label>
                <xforms:value>D</xforms:value>
            </xforms:item>
            <xforms:setvalue ev:event="xforms-select" ref="instance()/add"/>
        </xforms:select1>
    </xhtml:body>
</xhtml:html>