<!DOCTYPE my-root [
                      <!ELEMENT my-root  (worker*)>
                      <!ELEMENT agent  EMPTY>
                      <!ATTLIST agent
                          name  ID    #REQUIRED
                          boss  IDREF #REQUIRED>
                  ] >
<my-root>
    <agent name="Alice" boss="Alice"/>
    <agent name="Bob" boss="Alice"/>
    <agent name="Carole" boss="Alice"/>
    <agent name="Dave" boss="Bob"/>
</my-root>