Multiple XML files for one XSL file

I know there's been a few postings about this...

When I try to include another XML file, I get an error 'Root must be Root
Element' Error

<?xml version='1.0'?>
<xsl:stylesheet version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"

<xsl:template match="/">
     <xsl:apply-templates/>
</xsl:template>

<!-- Root Rule -->
<xsl:template match='order_info'>

     <!-- reference an external file -->
     <xsl:apply-templates select="document('external.xml')"/>

     <xsl:apply-templates select="order_info"/>
     <xsl:call-template name='splitline'/>

</xsl:template>

<!--
Match for the included XML file
This template is the root for processing the included XML
-->
<xsl:template match='included_xml'>
     <xsl:apply-templates select="doc_element"/>
</xsl:template>

</xsl:stylesheet>

INCLUDED XML - external.xml
<included_xml>
     <doc_element></doc_element>
</included_xml>


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