question about unique and xpath

Hi folks,

How can I achieve via "unique" that the element "id" is unique within the whole document ?

<root>
  
 <postition>
    <id>100</id>
  </postition>
  <postion>
    <id>101</id>
  </postion>
 
  <anotherpostion>
    <dummy>
      <id>102</id>
    </dummy>
  </anotherposition>

</root>
     
XML-Schema:

<element name="root">
  <complexType>
    <sequence>
      <element name="postion" maxOccurs="unbounded">
        <complexType>
          <sequence>
            <element name="id" type="positiveInteger"/>
          </sequence>
        </complexType>
      </element>
      <element name="anotherposition" maxOccurs="unbounded">
        <complexType>
          <sequence>
            <element name="dummy">
              <complexType>
                <sequence>
                  <element name="id" type="positiveInteger"/>
                </sequence>
              </complexType>
            </element>
          </sequence>
         </complexType>
        </element>
      </sequence>
    </complexType>
    <unique name="idMustBeUnique">
      <selector xpath=".//id"/>                       <------------- Is that one correct ?
      <field       xpath="."/>
    </unique>
  </element>

  Thanks in advance for an answer,

  Regards, Uwe.