<!DOCTYPE tv [ <!ELEMENT tv (date)*> <!ELEMENT date (#PCDATA)> <!ATTLIST date idref IDREF #IMPLIED id ID #IMPLIED>]>

<tv>
  <date idref=<error>"aaa"</error> id="bbb"/>
  <date idref="bbb"/>
  <date idref="ccc"/>
  <date id="ccc"/>
</tv>