LISTING 2: Using IDs and IDREFs to Represent the Authors in Listing 1

<bibliography>
  <author au_id="998-72-3567" wrote="PS2091 PS2106"
          au_lname="Ringer" au_fname="Albert" />
  <author au_id="899-46-2035" wrote="PS2091"
          au_lname="Ringer" au_fname="Anne" />

  <book title_id="PS2091" title="Is Anger the Enemy?"/>
  <book title_id="PS2106" title="Life Without Fear"/>
  <book title_id="TC7777" title="Sushi, Anyone?"/>
</bibliography>