declare entity in including document, use it in stylesheet

I have an xml document with a xml-stylesheet processing instruction.  I'm trying to declare an entity in the xml document and have the xsl recognize that entity.  Can anyone comment on whether this is possible in this situation?  I could accomplish the same by putting this info (a file name) in an xml field.  But for some reason, it seemed natural to do it this way.

TIA,



Travis


my xml document:
-----------
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE config [
  <!ENTITY % fileName SYSTEM "Config.xml">
  %fileName;
]>

<?xml-stylesheet type="text/xsl" href="editor.xsl"?>
<config>
 ...
</config>

and in the editor.xsl, I'm attempting to use the entity &fileName; with no luck.  Firefox gives the error: 
XML Parsing Error: undefined entity




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