<?xml version="1.0" standalone="no"?>
<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:func="http://exslt.org/functions"
 extension-element-prefixes="func"
 >
 <xsl:key name="mykey" match="test" use="func:initial(.)" />
  <func:function name="func:initial">
    <xsl:param name="s" />
    <func:result select="substring($s,1,1)" />
  </func:function>
</xsl:stylesheet>