定義の上書き
要素の定義はパラメータを使用
<!ELEMENT Set - O (SetInfo?, Book+, SetIndex?) >
<!ENTITY % book.class "Book">
<!ELEMENT Set - O (SetInfo?, (%book.class;)+, SetIndex?) >
Set = (SetInfo?, Book+, SetIndex?)
外部での定義の上書き
<!ENTITY % book.class "Article">
Set = (SetInfo?, Article+, SetIndex?)