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[@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>