If we wanted to be able to refer to that date description as described in the uniqueness example, just change it to a key.
This allows other elements in the document to refer to this date value.
The changed declaration:
<xs:element name="description-of-date"> <xs:complexType> <xs:sequence> <xs:element ref="xhtml:p" maxOccurs="unbounded"/> </xs:sequence> <xs:attribute name="date" type="xs:date" use="required"/> <xs:complexType> <xs:key name="date-key"> <xs:selector xpath="."/> <xs:field xpath="@date"/> </xs:key> </xs:element>