QXmlQuery: XSLT for-each-group not recognised

[alienmuppet]

The "QT Documentation":http://qt-project.org/doc/qt-4.8/xmlprocessing.html states that for-each-group is supported when using QXmlQuery. However I get an error when I try and use it:

"The element with local name for-each-group does not exist in XSL-T."

Am I missing something?

I am using Qt 4.8.4.
I am calling QXmlQuery like this:

@
QXmlQuery xmlQuery(QXmlQuery::XSLT20);
xmlQuery.setMessageHandler(&messageHandler);
xmlQuery.setFocus(&xmlBuffer);
xmlQuery.setQuery(&queryFile);
@

The line from my XSL file looks like this:

@
<xsl:for-each-group select="../readings/slot" group-by="@slot_dtm">
@

Thanks.