Xpath as variable - bis

Happy new year !

How can I solve the following problem (I tried several possibilities,
but none are successful) :

<xsl:template match="/(...)/xsl-File">
	<xsl:variable name="path" select="@XPath"/>
	<xsl:variable name="mainXML_node"
select="document('../XML/PM-EA-AA_IA_PA_TA.xml')"/>
	(...)
	<xsl:for-each-group select="concat($mainXML_node,$path)" <--
pseudo-code for understandig (...) group-by="@SAObjName">
	(...)

The xml processed contains an @Xpath attribute which contains the xpath
expression for processing another XML ($mainXML_node).

(the xml file)
(...)
<xsl_File name="App_Apt_Apx_Applications"
	file="App_Apt_Apx_Applications.xsl"

XPath="/Classes/Class/SADefinition[starts-with(@SAObjName,'app_') or
starts-with(@SAObjName,'apx_') or starts-with(@SAObjName,'apt_')]"/>
(...)

I tried several possibilities with the saxon:evaluate-node() function,
but unsuccessfully!

Thanks for your help

Cheers

Lawrence