Reference to an ENTITY

["Vines, John \(Civ, ARL/CISD\)" <jvines@arl.army.mil>]

Title: Reference to an ENTITY

All,
        I am working on a Schema and DTD for one of our data formats.  I need to include an "if/else" type statement for one of my elements.  The data can be stored directly in the XML file as CDATA or it can be stored in an external entity, an HDF5 file.

Here is the element I am working on:

<!--Lowest level element, describes the data that is present in the XDMF dataset-->
<!ELEMENT DataStructure (How do I get to my data?, XML(internal) or ENTITY(External)>
<!ATTLIST DataStructure
        Name CDATA #IMPLIED
        Rank CDATA #IMPLIED
        Dimensions CDATA #REQUIRED
        Precision (4 | 8) "4"
        Format (XML | HDF | Xml | Hdf) "XML"
        DataType (CHAR | FLOAT | INT | COMPOUND | Char | Float | Int | Compound) "Float"
>

        Again I'm new to XML and am not sure how to do this, so please forgive me if I ask the same type of question a couple of times. 

Thanks again for any help,

John Vines
ARL MSRC
HPCD, CSEB
Scientific Visualization Team