Grouping over multiple files

Hello

I am grouping <person> elements from four files (North, South, East & West).
The grouping is working fine, but I would like to add a source attribute to
the revenue elements to show which file they came from (please see 'desired
output' section below.

I have attached cut down versions of the input, along with the current
stylesheet and output.

Thanks.

--
Kevin

------------
North.xml
------------

<sales>
	<period id="October">
		<person>
			<name>Peter</name>
			<revenue>1000</revenue>
		</person>
		<person>
			<name>Fred</name>
			<revenue>2000</revenue>
		</person>
		<person>
			<name>Alex</name>
			<revenue>100</revenue>
		</person>
		<person>
			<name>Barney</name>
			<revenue>20</revenue>
		</person>
	</period>
</sales>

------------
South.xml
------------
<sales>
	<period id="October">
		<person>
			<name>Peter</name>
			<revenue>500</revenue>
		</person>
		<person>
			<name>Barney</name>
			<revenue>750</revenue>
		</person>
	</period>
</sales>

------------
East.xml
------------
<sales>
	<period id="October">
		<person>
			<name>Peter</name>
			<revenue>40</revenue>
		</person>
		<person>
			<name>Fred</name>
			<revenue>5000</revenue>
		</person>
		<person>
			<name>Alex</name>
			<revenue>100</revenue>
		</person>
	</period>
</sales>

------------
West.xml
------------
<sales>
	<period id="October">
		<person>
			<name>Fred</name>
			<revenue>400</revenue>
		</person>
		<person>
			<name>Alex</name>
			<revenue>2000</revenue>
		</person>
	</period>
</sales>

------------
Sales.xsl
------------
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
	<xsl:output indent="yes" method="xml"/>

	<xsl:variable name="northsales"
select="document('north.xml')/sales/period/person"/>
	<xsl:variable name="southsales"
select="document('south.xml')/sales/period/person"/>
	<xsl:variable name="eastsales"
select="document('east.xml')/sales/period/person"/>
	<xsl:variable name="westsales"
select="document('west.xml')/sales/period/person"/>

	<xsl:template match="/">
		<combinedsales>
			<xsl:for-each-group select="$northsales,
$southsales, $eastsales, $westsales" group-by="name">
				<person>
					<xsl:copy-of
select="(current-group()/name)[1]"/>
					<xsl:for-each
select="current-group()/revenue">
						<xsl:copy-of select="."/>
					</xsl:for-each>
				</person>
			</xsl:for-each-group>
		</combinedsales>
	</xsl:template>
</xsl:stylesheet>

------------
Current Output
------------
<combinedsales>
  <person>
    <name>Peter</name>
    <revenue>1000</revenue>
    <revenue>500</revenue>
    <revenue>40</revenue>
  </person>
  <person>
    <name>Fred</name>
    <revenue>2000</revenue>
    <revenue>5000</revenue>
    <revenue>400</revenue>
  </person>
  <person>
    <name>Alex</name>
    <revenue>100</revenue>
    <revenue>100</revenue>
    <revenue>2000</revenue>
  </person>
  <person>
    <name>Barney</name>
    <revenue>20</revenue>
    <revenue>750</revenue>
  </person>
</combinedsales>

------------
Desired Output
------------
<combinedsales>
  <person>
    <name>Peter</name>
    <revenue source="north.xml">1000</revenue>       <!-- add @source to
indicate file origin -->
    <revenue source="south.xml">500</revenue>
    <revenue source="east.xml">40</revenue>
  </person>
  <person>
    <name>Fred</name>
    <revenue source="north.xml">2000</revenue>
    <revenue source="east.xml">5000</revenue>
    <revenue source="west.xml">400</revenue>
  </person>
  <person>
    <name>Alex</name>
    <revenue source="north.xml">100</revenue>
    <revenue source="east.xml">100</revenue>
    <revenue source="west.xml">2000</revenue>
  </person>
  <person>
    <name>Barney</name>
    <revenue source="north.xml">20</revenue>
    <revenue source="south.xml">750</revenue>
  </person>
</combinedsales>