<?xml version="1.0"?> 
<xsl:stylesheet version="1.0" 
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:pre="http://www.example.com">

  <xsl:template match="/">
    <!-- unavailable extension function is a must signal error;
         processors cannot recover -->
    <root><xsl:value-of select="pre:fn()"/></root>
  </xsl:template>

</xsl:stylesheet>
