More Complex Example of Aggregating Modifiers in DTDs

Following is a more complicated example. Suppose you want book elements to include

Exactly one title

Either an author or an editor, but it is okay if neither appear

Zero or more paragraphs

To accomplish this, you would perform steps that generate the following tree representation:


book Sequence title Optional Choice author editor Zero or More paragraph

In the Text view of the DTD, the definition for the book element is as follows:


<!ELEMENT book (title, (author\|editor)?, paragraph*)>