Schema error or validation bug ?

The following construct make xerces complains:
<complexType name="fooType">
  <complexContent>
    <extension base="model:barType">
      <all>
        <element name="elmt1" type="string"/>
      </all>
    </extension>
  </complexContent>
</complexType>

<complexType name="barType">
  <complexContent>
    <extension base="model:fooType">
      <all>
        <element name="elmt2" type="string"/>
        <element name="elmt3" type="string"/>
      </all>
    </extension>
  </complexContent>
</complexType>

[Error] file.xml:6:4: Schema error: ComplexType 'barType': 
cos-all-limited.1.2:  An "all" model group that is part of a complex type 
definition must constitute the entire {content type} of the definition.

Does it means inheriting type with unordered elements is illegal, or is it a 
xerces bug ?
-- 
Guillaume Rousse <rousse@ccr.jussieu.fr>
GPG key http://bohm.snv.jussieu.fr/~rousse/gpgkey.html