Xalan question

I am using Xalan J Version 2.5.2. I want to run the following stylesheet:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">
  <xsl:output method="xml" omit-xml-declaration="yes"/>
  <xsl:template match="book">  
    <xsl:variable name="a"><xsl:number/></xsl:variable>
    <xalan:write file="{concat($a,'.xml')}">
      <xsl:copy-of select="*" />
    </xalan:write>
  </xsl:template>
</xsl:stylesheet>

I run Xalan with the following commandline:

set CLASSPATH=D:\xalan.jar;D:\bsf.jar
java org.apache.xalan.xslt.Process -IN input.xml -XSL SplittingUp.xslt

I get as an output: The prefix "xalan" for element "xalan:write" is not
bound.

What does that mean? Does somebody here use Xalan for processing with XSLT
extension. I am despeatly looking for a solution....

Karl


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