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Chapter 14. Creating XML Document Type D... > Optional Content Models and Ambiguit...

Optional Content Models and Ambiguities

When you review a DTD you are working on, take a close look at the occurrence indicators. If a lot of optional elements exist in a content model, this will often be an indication that something is wrong with the model.

An example of this comes from the SGML world of books. Consider the back part of a book. You might create an XML DTD for a book, for instance, and create a container element BACK to make it easier to process the parts together. The BACK element consists, as books often do, of any number (including zero) of appendixes, followed by an optional glossary and then an optional index. When you can describe a content model in plain English such as the following, it is quite easy to model:


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