５. 実装
29/31

 サンプルＤＴＤ

   <!ELEMENT books (book)+>
   <!ELEMENT book (title,publish_year,author)>
   <!ELEMENT title (#PCDATA)>
   <!ELEMENT publish_year (#PCDATA)>
   <!ELEMENT author (name,others?)>
   <!ATTLIST author country CDATA #REQUIRED>
   <!ATTLIST author sex CDATA #REQUIRED>
   <!ELEMENT name (#PCDATA)><!ELEMENT others (#PCDATA)>