## 29.4 Document Type Definitions (cont.)

- Expressions can be combined:

  > `<!ELEMENT section (title, (paragraph | (image, title?))+)>`

- Defines an element `section` whose children are:
  1. A `title` element
  2. A sequence of one or more of:
     - `paragraph` elements
     - `image` elements followed by optional `title` elements
- Avoid mixed content for DTDs
- Avoid `ANY` type in DTDs

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