[Exist-open] Operation on sorted sequence not supported?

[Jonas L. <my....@gm...>]

I wish to sort some items, and then group them by date (for instance
2005-12-29). Then I want to process the group further. However, the
xquery below causes the following exception:

org.exist.xquery.XPathException: Operation not supported
=09at org.exist.xquery.value.OrderedValueSequence.toNodeSet(OrderedValueSeq=
uence.java:138)
=09at org.exist.xquery.LocationStep.eval(LocationStep.java:205)
=09at org.exist.xquery.AbstractExpression.eval(AbstractExpression.java:56)
=09at org.exist.xquery.PathExpr.eval(PathExpr.java:196)
        .....

What operation is not supported? If this operation is not allowed,
then how can I achieve what I want? Ideas?

Regards
Hans

.....

xquery version "1.0";

let $doc :=3D (
<root>
<raw>2005-12-29T18:49:29.859+01:00</raw>
<raw>2005-12-29T18:51:17.14+01:00</raw>
<raw>2005-12-29T19:01:39.5+01:00</raw>
<raw>2005-12-30T01:05:38.281+01:00</raw>
<raw>2006-01-02T00:54:57.968+01:00</raw>
<raw>2006-01-13T00:26:01.906+01:00</raw>
</root>)

let $sorted :=3D
(for $item in $doc/* order by $item/entry/raw
return
$item)

return (<page>{
         for $subset in distinct-values($sorted/raw/substring(text(), 1, 10=
))
         let $set :=3D $sorted/raw[substring(text(), 1, 10) =3D $subset]
                                  order by $subset descending
                 =09          return <sequence><s>{$subset}</s>{$set}</sequ=
ence>
}</page>)