DTD attribute list syntax for referencing content

[Dan Matthews <dan@grandprix.freeserve.co.uk>]

I am trying to validate an XML file against a DTD, and enforce an element 
attribute which contains data from the XML content (i.e. a reference to the 
XML file content).

here's my DTD code so far:

<!ELEMENT object (#PCDATA )>
<!ATTLIST object source CDATA #FIXED '{@object}'>

As you can see, I want to enforce a 'source' attribute for the object element, 
and populate it with the content from, for example, the object element itself. 
I just can't get info on referencing non-string info in a #FIXED attribute 
declaration! Please help - i'm desperate to get this working!!!

TIA,
dan