[oXygen-user] Using xs:unique

[Olumide]

I'd appreciate help figuring out why the schema below validates the 
following xml file even though it the id='3' is not unique.

Regards,

- Olumide

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<?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:unique name="uniqueId-a">
               <xs:selector xpath="*"/>
               <xs:field xpath="@id"/>
             </xs:unique>
           </xs:element>

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

             <xs:unique name="uniqueId-b">
               <xs:selector xpath="*"/>
               <xs:field xpath="@id"/>
             </xs:unique>
           </xs:element>

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


----------------------------------------------------

<?xml version="1.0" encoding="UTF-8"?>
<Item>
     <a id="3"/>  <!-- shared id -->
     <a id="2"/>
     <b id="3"/>  <!-- shared id -->
     <a id="4"/>
     <b id="0"/>
     <b id="3"/>  <!-- shared id -->
</Item>