nested indexterms

I recently hit a problem with nested indexterms.  While nesting an indexterm within an indexterm is invalid it seems there is a loop hole.

If you put the systemitem tag within the primary tag then it allows an additional indexterm which is valid in a systemitem tag.

I have not tried other tags.

<indexterm>
  <primary>
   <systemitem>systemitem
     <indexterm>
       <primary>nested indexterm</primary>
     </indexterm>
    </systemitem>
  </primary>

</indexterm>

The validation process is not catching this in Arbortext or saxon.  
I build html and pdf output and it is only the PDF that is catching this with just a message but not breaking the build.

 java:         [error] no entries for index key nested indexterm'

I would like to address this in the DTD so the writers can catch this when validating but do not know where or how to state in the DTD to not allow nested indexterms once you nesting tags.

I'm using Docbook 4.0 and I do have some customizations already so it is not a problem to add to my custom DTD.

Can someone help?

Thanks,
Joy

-- 
Joy Andree
Software Developer II

ANSYS, Inc.

275 Technology Drive
Canonsburg, PA 15317
Tel: 724-514-2944
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