string replace probleme

"maalej mustapha" <[email protected]>
Newsgroups gmane.text.xml.xslt.extensions
Message-ID <[email protected]>
hi 

this is my xsl :
.....
<xsl:variable name="test">aaaaa</xsl:variable>
<xsl:value-of select="str:replace($test,'a','b')"/>
....
I become always a error message.
why take the function not a  variable($test).

can you help me plz

maalej
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