Re: xsl namespace question

I am attempting to use EXSLT's str:tokenize('foo', ''), in that format.

At the top of my document I have:

<xsl:stylesheet version='1.0' xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
        xmlns:str="../../Includes/exslt/str.xsl">

I replaced it with the suggestion on the site
(xmlns:str="http://exslt.org/strings") because I get an error about no
functions being at that location. However, I'm getting the same error.

Although there are no actual functions in that file, it of course
imports several files full of functions. Including the individual page
yielded no results, as well.

Is there something obvious I'm missing? I admittedly have no clear
grasp on namespaces, but reading hasn't yielded anything. Sometimes it
seems its a location, at other times it seems to point to some other
declaration.

-Steve