[oXygen-user] Checking unique Element Ids

[Olumide]

Is it possible to use XML 1.1 schema to verify that that elements in a 
document contain unique Ids. See For example the attached test XML 
document and Schema.

Thanks.

-------------- XML Document --------------

<?xml version="1.0" encoding="UTF-8"?>
<Item>
	<c id="0"/>
	<a id="1"/>
	<a id="2"/>
	<b id="3"/>
	<a id="4"/>
	<b id="0"/>
	<d id="6"/>
	<d id="7"/>
	<b id="3"/>
	<c id="1"/>
</Item>

-------------- XML 1.1 Schema --------------

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
elementFormDefault="qualified">
     <xs:element name="Item">
         <xs:complexType>
             <xs:sequence maxOccurs="unbounded">
                 <xs:choice>

                     <xs:element name="a">
                         <xs:complexType>
                             <xs:attribute name="id" use="required"/>
                         </xs:complexType>
                     </xs:element>

                     <xs:element name="b">
                         <xs:complexType>
                             <xs:attribute name="id" use="required"/>
                         </xs:complexType>
                     </xs:element>

                     <xs:element name="c">
                         <xs:complexType>
                             <xs:attribute name="id" use="required"/>
                         </xs:complexType>
                     </xs:element>

                     <xs:element name="d">
                         <xs:complexType>
                             <xs:attribute name="id" use="required"/>
                         </xs:complexType>
                     </xs:element>

                 </xs:choice>
             </xs:sequence>
         </xs:complexType>
     </xs:element>
</xs:schema>