gistfile1.xml

<xhtml:html xmlns:exf="http://www.exforms.org/exf/1-0" xmlns:xhtml="http://www.w3.org/1999/xhtml"
            xmlns:xforms="http://www.w3.org/2002/xforms">
    <xhtml:head>
        <xforms:model id="model">
            <xforms:instance id="instance">
                <instance xmlns="">
                    <record code="c"/>
                </instance>
            </xforms:instance>
        </xforms:model>
    </xhtml:head>
    <xhtml:body>
        <xforms:output id="output" ref="exf:sort(record[@code = 'c'], '@code', 'text', 'ascending', 'lower-first')"/>
    </xhtml:body>
</xhtml:html>

This is a reproducible sample for [ #315140 ] exf:sort() can cause "invalid slot for variable" error in Saxon