Adding attributes to anyType

Some colleagues are proposing a schema within which attributes are added to
xs:anyType using the XSD extension mechanism. 
For example 
 
<complexType name="NewType">
  <complexContent>
    <extension base="anyType">
      <attribute name="att1" type="string"/>
    </extension>
  </complexContent>
</complexType>

I don't see how this makes sense, since anyType is an infinite set, so all
specific content models must be derived by restriction. 
But Xerces (in oXygen) doesn't complain. 
 
Is Xerces wrong or am I mistaken?

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Simon Cox

European Commission, Joint Research Centre 
Institute for Environment and Sustainability 
Spatial Data Infrastructures Unit, TP 262 
Via E. Fermi, 2749, I-21027 Ispra (VA), Italy 
Tel: +39 0332 78 3652 
Fax: +39 0332 78 6325 
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Any opinions expressed are personal unless otherwise indicated.