splitting into multiple files and using a lookup xsl:k

Hi.

I'm rather new to XSL and am having some difficulty understanding how to use
the xsl:key function.  I'm sorry if this is a FAQ, but I've tried searching
the archives and couldn't find the answer to my problem.

CONTEXT: I'm trying to split up a large XML file (bigFile.xml) into multiple
smaller files.  The file will be splitting up at the "<association>" tags
(see below xml example).

PROBLEM: I have to generate the filename using the value of one of the
elements (countyName tag, e.g. Alameda) in the <association> branch of the
tree.  I need to reference those county names to lookup and return a
corresponding county number: e.g. Alameda would return number 1, Alpine
would return number 2, etc...  I can't simply use the position() function,
since data from some counties may be missing.

I initially tried to use a bunch of xsl:choose & xsl:when statements (ugly
and inelegant as it is), but that wasn't working very well, so I thought I'd
try something else.  I've tried to incorporate the xsl:key statement (see
XSLT fragment below) into this but it doesn't seem to be working.

Can anyone out there offer any advice / suggestions on using the xsl:key
statement (or another approach) to accomplish my objective?

Any advice would be MUCH appreciated.

TIA,
Helen Kim

________________________________________________
input XML fragment, bigFile.xml:
.....
  <title class="SystemTitle" toc-level="1">TableData</title>
  <association>
  	<countyName>Alameda</countyName>
  	<output-object type="table" class="Table">
 	 <!--elided table data-->
 	</output-object>
  </association>
 <association>
  	<countyName>Alpine</countyName>
 	<output-object type="table" class="Table">
 	 <!--elided table data-->
 	</output-object>
  </association>
  <association>
  	<countyName>Amador</countyName>
 	<output-object type="table" class="Table">
 	 	<!--elided table data-->
 	</output-object>
  </association>
<!--(etc... for all 58 counties)-->
.....

_____________________________________________
XSLT fragment:

!--Set up a variable to hold contents of the counties.xml lookup -->
<xsl:variable name="countyLookup-top"
select="document('counties.xml')/counties"/>

<!--define key-->
<xsl:key name="countyLookup" match="name" use="num" />

<xsl:template match="/">
	<xsl:apply-templates select="key('countyLookup', 'Alpine')" />
</xsl:template>

<xsl:template match="name">
	<xsl:variable name="countyNo">
		<xsl:value-of select="." />
	</xsl:variable>
</xsl:template>

<xsl:template match="association">

	<xsl:variable name="filenameFrag" select="//proc/title" />
	<xsl:variable name="file" select="concat($filenameFrag, '_', $countyNo,
'.xml')" />

	<!--creating new file for each association element processed-->
	<xsl:document href="e:\data\{$file}">
		<xsl:copy>
			<xsl:element name="association">
				<xsl:attribute name="fileFragment" >
					<xsl:value-of select="//proc/title" />
				</xsl:attribute>
				<xsl:attribute name="countyNum" >
					<xsl:value-of select="$countyNo" />
				</xsl:attribute>
			</xsl:element>
			<!------etc...-->
___________________________________________________________________________
XML "lookup", counties.xml:

<counties>
	<county><name>Alameda</name><num>1</num></county>
	<county><name>Alpine</name><num>2</num></county>
	<county><name>Amador</name><num>3</num></county>
	<county><name>Butte</name><num>4</num></county>
	<county><name>Calaveras</name><num>5</num></county>
	<!------etc...-->
</counties>




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