XML Schema question on key, keyref constraints

[Murali Mani <mani@CS.UCLA.EDU>]

I have a question on XML-Schema:

Is it possible to have:

<xs:element name="vehicle">
  <xs:complexType>
    <xs:attribute name="plateNumber" type="xs:integer"/>
  </xs:complexType>
</xs:element>

<xs:element name="state">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="code" type="xs:string"/>
      <xs:element ref="vehicle" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:key name="reg">
    <xs:selector xpath=".//vehicle"/>
    <xs:field xpath="@plateNumber"/>
  </xs:key>
</xs:element>

<xs:element name="person">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="car" maxOccurs="unbounded"/>
        <xs:complexType>
          <xs:attribute name="regPlate" type="xs:integer"/>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
  </xs:complexType>

  <xs:keyRef name="carRef" refer="reg">
    <xs:selector xpath=".//car"/>
    <xs:field xpath=@regPlate"/>
  </xs:keyRef>
</xs:element>

Is the above a correct schema. And in that case can I have a document like

<root>
  <state>
    <code>CA</code>
    <vehicle plateNumber="1"/>
    <vehicle plateNumber="2"/>
  </state>
  <state>
    <code>MA</code>
    <vehicle plateNumber="1"/>
  </state>

  <person>
    <car regPlate="1"/>
  </person>
  <person>
    <car regPlate="2"/>
  </person>
</root>

is this a valid instance for that schema??

thanks, murali.