Suppose in paragraph text we want to be able to have embedded dates link to date descriptions.

We can add a 'dateref' element that has date content.

We'll enforce that the date content has to have a corresponding 'description-of-date' element.

The 'dateref' declaration:

<xs:element name="dateref" type="xs:date">
  <xs:keyref name="date-pointer" refer="my:date-key">
    <xs:selector xpath="."/>
    <xs:field xpath="."/>
  </xs:keyref>
</xs:element>