dynamic grouping assignment

Hi,
How can I dynamically assing a grouping key, where I provide some of the 
grouping keys as parameters.

I have the parameters

  <xsl:param name="groupingKeys" >
        <key>order_typ</key>
        <key>ordr_block</key>
    </xsl:param > 

I would like to add to the group-by evaluation.

 <xsl:for-each-group select="result" group-by="concat(receiver, operation, 
data, prev_bus_state, string-join(for $x in $groupingKeys return $x/key, 
','))">

I guess I need to use the saxon:evaluate() extension, but I couldn't get 
it right.

thanks


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