Parentheses combine several elements into a single element.
Parenthesized elements can be nested inside other parentheses in place of a single element.
The parenthesized elements can be suffixed with a plus sign, a comma, or a question mark.
<!ELEMENT dl (dt, dd)*>
<!ELEMENT ARTICLE (TITLE, (P | PHOTO | GRAPH | SIDEBAR | PULLQUOTE | SUBHEAD)*, BYLINE?)>
<ELEMENT catalog (category, cataloging_info, last_updated, copyright, maintainer, (composer | composition)*)>