dynamic file variable

I have a main.xml file, in which there is a lot of
<ref> elements.
There is an attribute "linkname" in each <ref> like:
<ref linkname="test_2.xml"/>

In test_2.xml, there is only one root element and a
child element like:
<test>
      <sub_test link="html_test.html"/>
</test>

I use the following code to get the referenced data:
<xsl:template match="ref">
<xsl:variable name="refname">
       <xsl:value-of select="@refname"/>
</xsl:variable>
<br/>
Here is the referenced file data:
<xsl:value-of 
select="document($refname)/test/sub_test/@link"
/>
</xsl:template>

It returns null.  However, if the variable "$refname"
is replaced by the actual file name "test_2.xml", then
the data can be properly grabbed.

Any one can tell me what is wrong?

Thanks so much




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