ID and IDREF Attributes

Similar to anchors in HTML

<!ELEMENT book (title,author,publisher)>
<!ATTLIST book destination ID #REQUIRED>
 
<!ELEMENT bestbooks (bookref)* >
<!ELEMENT bookref (#PCDATA)>
<!ATTLIST bookref target IDREF #IMPLIED>


So a book might be:

  <book destination="BookId28752">
    <title>Introduction to GKS</title>
    <author>Bob Hopgood</author>
    <publisher>Academic</publisher>
  </book>

And a reference to it might be:

  <bestbooks>
    <bookref target="BookId28752">Introduction to GKS</bookref>
    <bookref>GKS for Dummies</bookref>
  </bestbooks>