Setting the decimal separator in fr:number with a nested element

gistfile1.xml

<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>
        <xh:title>XForms Hello</xh:title>
        <xf:model>
            <xf:instance>
                <value separator=";">1234.5678</value>
            </xf:instance>
        </xf:model>
    </xh:head>
    <xh:body>
        <fr:number ref="." digits-after-decimal="4">
            <fr:decimal-separator ref="@separator"/>
        </fr:number>
    </xh:body>
</xh:html>