XSL: Nested stylesheets (namespace alias) problem

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" 
	xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
	xmlns:sub="http://www.w3.org/1999/XSL/TransformAlias"
	xmlns:exslt="http://exslt.org/common"
	exclude-result-prefixes="xsl php exslt">
 
	<xsl:output method="html" media-type="text/html"
		indent="yes"
		omit-xml-declaration="yes"
		encoding="ISO-8859-1"/>
		
	<xsl:namespace-alias stylesheet-prefix="sub" result-prefix="xsl" />
 
	<xsl:template name="callme">
	
			<h1>Welcome</h1>
 
		<sub:stylesheet version="1.0">
 
			<sub:template match="/">
				<!-- Setting a param in this "stylesheet" to see if we can access it later -->
				<sub:param name="testval" select="'Yes, It worked.'"/>
				<sub:call-template name="test" />
			</sub:template>
 
			
			<sub:template name="test">
				Val: <sub:value-of select="$testval"/>
			</sub:template>
 
		</sub:stylesheet>
		
	</xsl:template>
 
</xsl:stylesheet>