[xsl] XSLT merge documents into variable

Bit of a newbie here. I want do some quite simple file processing. I have a 
basic source file with file references in it. These files should be merged into 
a variable, which can then be further processed; in my example below the 
@amount of each line[(_at_)ref] is summed. But I'm already stuck in the first 
part of this, how do I correctly put file1.xml and file2.xml into a variable 
which then contains (psuedo) 
"<root><data><lines/></data>><data><lines/></data></root>"? And how can I then 
use that variable as a basis for further processing?

I'm using XSLT 1.0.

<!-- To XML to process -->
<?xml version="1.0"?>
<files>
        <file href="file1.xml"/>
        <file href="file2.xml"/>
</files >

<!-- file1.xml -->
<data>
        <line ref="A" amount="2"/>
        <line ref="B" amount="3"/>
</data>

<!-- file2.xml -->
<data>
        <line ref="A" amount="4"/>
        <line ref="B" amount="8"/>
</data>

<!-- result should be -->
<data>
        <line ref="A" amount="6"/>
        <line ref="B" amount="11"/>
</data>

<!-- XSLT -->

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet
        xmlns="http://www.w3.org/1999/xhtml";
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
        xmlns:exslt="http://exslt.org/common";
        version="1.0">

        <xsl:output method="xml" omit-xml-declaration="no" indent="yes"/>
        
        <xsl:templates match="files">
                <xsl:variable name="m">
                        <xsl:apply-templates select="document(file/@href)"/>
                </xsl:variable>
                <xsl:apply-templates select="exslt:node-set($m)" mode="merge"/>
        </xsl:templates>
        
        <xsl:template match="data" mode="merge">
                <root>
                        <xsl:copy-of select="."/>
                </root>
        </xsl:template>

</xsl:stylesheet>



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