thank you Martin but but when you wrote

To define your own functions you are free to choose a >namespace but you need to choose one.

i don't know how to declare my own functions.

<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:f="http://example.com/f"
  exclude-result-prefixes="f"
  version="2.0">

	Martin Honnen
	http://msmvps.com/blogs/martin_honnen/