An attempt to understand xsl:key and the key() function and why it is not possible to insert the keys into the document itself.

key.xsl

<?xml version="1.0" encoding="UTF-8"?>
<!--
	Attempt to use xsl:key on data not stored in the document itself.

	We _can_ load external XML into a variable using document() and use
	the key() function on that.

	We _cannot_ include the XML into our stylesheet and use the key()
	function on that.

	The reason seems to be that using select="" will give us a variable
	containing a set of nodes while other ways return result tree fragments
	which cannot be used with further select statements.

	A workaround is to use the node-set() function for converting the result
	tree fragment to a node set. It is not part of XSL 1.

	http://stackoverflow.com/a/5303159

	The example uses for-each on the document to make it the context node
	as hinted in:

	http://www.oxygenxml.com/archives/xsl-list/200105/msg00546.html
-->
<xsl:stylesheet
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    version="1.0"
	>
	
	<xsl:key name="preg" match="person" use="@id"/>		

	<xsl:template match="/">
		<output>
			<xsl:for-each select="document('persons.xml')">
				<xsl:value-of select="key('preg', '050676')/@name"/>
			</xsl:for-each>
		</output>
	</xsl:template>

</xsl:stylesheet>

persons.xml

<?xml version="1.0" encoding="UTF-8"?>
<persons>
	<person name="Tarzan" id="050676"/>
	<person name="Donald" id="070754"/>
	<person name="Dolly" id="231256"/>
</persons>

Thanks! found this extremely helpful - was struggling to get a good example that worked w/xslt1.0...The "key" understanding that your code helped me with was that 'key' is a "query function", not a set of values.