Creating multi-level groupings

I am trying to create a HTTP table using the following data, however, I want it grouped on multiple fields (namely "product", "doc_type" & "customer").

I've manage to group on one field (see bottom of mail) but can't find info on extending this to more than one key.


--------------------------Start of XML ------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="VitalVolumes4.xsl"?>
<!-- Vital Document Volumes Database -->
<volume>
	<product>01 Vital ABC
		<doc>
			<doc_type>02 External Reqs</doc_type>
			<customer>Government</customer>
			<id>3246.01.02.01.01</id>
			<title>Specification Document for the FR trial</title>
		</doc>			
		<doc>
			<doc_type>02 External Reqs</doc_type>
			<customer>NASA</customer>
			<id>3246.01.02.02.01</id>
			<title>Placeholder for T-Mobile Doc</title>
		</doc>
	</product>
	<product>02 Vital XYZ
		<doc>
			<doc_type>01 Type</doc_type>
			<customer>Customer A</customer>
			<id>3246.02.01.01.01</id>
			<title>CSD for Customer A - Vital XYZ</title>
		</doc>			
		<doc>
			<doc_type>05 Another Type</doc_type>
			<customer>Customer B</customer>
			<id>3246.02.05.01.01</id>
			<title>Some other document</title>
		</doc>
	</product>
</volume>
----------------End of XML-----------------------------------

----------------Start of XSL--------------------------------
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:key name="docs_by_doc_type" match="doc" use="doc_type"/>
..
..
<xsl:for-each select="doc[count(. | key('docs_by_doc_type', doc_type)[1]) = 1]">
			<xsl:sort select="doc_type"/>
			<tr>
				<td colspan="4">
					<xsl:value-of select="doc_type"/>
				</td>
			</tr>
			<xsl:for-each select="key('docs_by_doc_type', doc_type)">
				<xsl:sort select="id"/>
				<tr>
					<td/>
					<td>
						<xsl:value-of select="id"/>
...
.
.
.
----------------End of XSL-----------------------------------