question about xpath function concat

> I am trying to learn Orbeon by porting an existing
> Cocoon app.
> I am using a file named FormUtilities.xsl.
> My app generates the Orbeon error:
>
> XPath syntax error at char 43 on line 23 in
> {...agePrefix, 'header_title.gi...}: Cannot find a
> matching 2-argument function named
> {http://www.w3.org/2005/02/xpath-functions}concat()
>
> The relevant code section that contains the use of
> concat in FormUtilities.xsl is at the end of this
> message.  Does anyone know if I am supposed to add a
> .jar file to make this app work?
> If so what is it and where do I put it?
>
> xmlns:fn="http://www.w3.org/2005/02/xpath-functions"
> …
>     <xsl:output method="html" version="4.0"
> encoding="ISO-8859-1" indent="yes"
>         doctype-public="-//W3C//DTD HTML 4.01
> Transitional//EN"/>
>
> <!-- Logo shared by all pages -->
> <xsl:template name="generate-header">
> <xsl:param name="imagePrefix"/>
> <xsl:param name="numCols" as="xs:integer"
> select="2"/>
> <tr>
> <td height="0">
> <xsl:attribute name="colspan">
> <xsl:value-of select="$numCols"/>
> </xsl:attribute>
> <xsl:element name="img">
> <xsl:attribute name="src">
> <xsl:value-of select="fn:concat($imagePrefix,
> 'header_title.gif')"/>
>
>
>
>
>
> __________________________________________________
> Do You Yahoo!?
> Tired of spam?  Yahoo! Mail has the best spam protection around
> http://mail.yahoo.com 
>
>  
> ------------------------------------------------------------------------
>
>
> --
> You receive this message as a subscriber of the [hidden email] mailing list.
> To unsubscribe: mailto:[hidden email]
> For general help: mailto:[hidden email]?subject=help
> ObjectWeb mailing lists service home page: http://www.objectweb.org/wws
>