DTD Question - PCDATA followed by an optional element

[Norman H. Samuelson]

I have a large number of fields that normally hold simple textual data, but 
might also include a comment.  The XML would look like this with no comment:

   <rho0> 1.0 </rho0>

or this, when the comment is present:

   <rho0> 1.0 <comment> blah, blah </comment> </rho0>

I have no problem in the DTD when I leave out the possibility of a comment, 
I simply use something like this:

   <!ELEMENT rho0 (#PCDATA)>

To include the comment, I would like to be able to simply say:

   <!ELEMENT comment (#PCDATA)>
   <!ELEMENT rho0 (#PCDATA, comment?)>

That doesn't seem to be acceptable.  I also tried:

   <!ELEMENT rho0 ((#PCDATA) | (#PCDATA, comment))>

Is it possible to do what I want?

If not, I could consider putting one more level of tags around the data, 
like this:

   <rho0> <data> 1.0 </data> <comment> blah, blah </comment> </rho0>

But that adds extra bulk, and code, that should not be needed.  Any 
suggestions would be appreciated.

Whatever solution I come up with must work with IBM's XML4J parser.


- Norm -

Norman H. Samuelson                  nhs at llnl.gov
Lawrence Livermore National Lab      925-422-0661
P.O. Box 808, L-98
Livermore, CA 94551 

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