declaration with 'type' attribute must not have nested type declaration

I don't understand this error (can we not have our own ds:type element or attribute?); which part of the schema spec addresses this?

http://www.w3.org/2000/06/webdata/xsv?docAddrs=http%3A%2F%2Fpolicy.w3.org%2FWD-xmldsig-core-20000711%2Fsignature-example.xml+http%3A%2F%2Fpolicy.w3.org%2FWD-xmldsig-core-20000711%2Fxmldsig-core-schema.xsd&keepGoing=on&style=msxsl

http://policy.w3.org/WD-xmldsig-core-20000711/xmldsig-core-schema.xsd:148:11: Error: declaration with 'type' attribute must not have nested type declaration


 <element name="RetrievalMethod">
  <complexType content="elementOnly">
   <sequence minOccurs="1" maxOccurs="1">
     <element name="Location" type="uriReference" minOccurs="1" maxOccurs="1" /> 
     <element name="Method" type="string" minOccurs="1" maxOccurs="1" /> 
     <element ref="ds:Type" minOccurs="1" maxOccurs="1" /> 
     </sequence>
    <attribute name="Encoding" type="uriReference" use="optional" /> 
   </complexType>
  </element>

 <element name="Type" type="string">
  <complexType content="mixed">
   <any namespace="##any" minOccurs="1" maxOccurs="unbounded" /> 
   <attribute name="Encoding" type="uriReference" use="optional" /> 
   </complexType>
  </element>


_________________________________________________________
Joseph Reagle Jr.   
W3C Policy Analyst                mailto:reagle@w3.org
IETF/W3C XML-Signature Co-Chair   http://www.w3.org/People/Reagle/