libxml2-2.7.7/doc/examples/test2.xml

<!DOCTYPE doc [
<!ELEMENT doc (src | dest)*>
<!ELEMENT src EMPTY>
<!ELEMENT dest EMPTY>
<!ATTLIST src ref IDREF #IMPLIED>
<!ATTLIST dest id ID #IMPLIED>
]>
<doc>
  <src ref="foo"/>
  <dest id="foo"/>
  <src ref="foo"/>
</doc>