Hi Stefan,

I would recommend looking into using a schema language (XML Schema,
Relax NG) instead of a DTD.
With DTD probably the best approach is to use parameter entities to
define the content model for elements and the available attributes. You
can look at DocBook 4 for an example of using parameter entities. Note
that the next version of DocBook, DocBook 5 will provide a Relax NG
schema. More, it should be possible to convert from schema to DTD so if
you need to provide also a DTD then you will be able to do so. DocBook
5 for instance will provide also a DTD and an XML Schema.

Best Regards,
George
---------------------------------------------------------------------
George Cristian Bina
<oXygen/> XML Editor, Schema Editor and XSLT Editor/Debugger
www.---.com


stefan.rickfjord@g... wrote:
> Hi,
>
> I am a newbie to programming using XML and I'm currently working on a
> side-project which will house some database information in a XML-file.
> I am writing a DTD to specify the XML-format, mostly for the purpose of
> documenting the XML database.
>
> My issue is that I have some fields, e.g.
> <!ELEMENT Vehicle ( Car? )>
> <!ELEMENT Car ( Gasoline | Diesel )>
> <!ELEMENT Gasoline EMPTY>
> <!ELEMENT Diesel EMPTY>
>
> but I want to make sure that this database is extendable. I.e. after
> using the database for a while in my application I want to be able to
> add stuff that I come up with later on, e.g.
> <!ELEMENT Motorcycle ( Sportbike | Harley )>
> <!ELEMENT Sportbike EMPTY>
> <!ELEMENT Harley EMPTY>
>
> I'd rather not add a lot of "Reserved"-elements in the DTD, as this is
> not a nice solution. My own little idea is that i'm anyway able to add
> items to the DTD further on since I both have the DTD of the XML and
> the source of the application using it. Is this correct?
>
> I've looked around and found another way to do this and that is to
> include the old DTD as an entity, is that the way to go?
>
> Please excuse my newbie-question, I just want to know if my way of
> thinking is correct.
> 
> Thanks a lot for reading!