xml schema question

[Peter Menzel]

Hi all,
i am working on a xml schema and have the following problem, maybe 
somebody can help me.

i would like to have the follwoing elment with content:
<element>
   <aa/>
   <cc/>
   <bb/>
</element>
The elements aa and bb are required to be there and they may occur just 
one time. Other sub elements of element can occur many times. Especially 
the order of the elements is random, so i may not use xsd:sequence.

My xml schema snippet for this element:
<xsd:element name='element'>
  <xsd:complexType>
      <xsd:choice>
           <xsd:element ref='dd' minOccurs='0' maxOccurs='unbounded'/>
           <xsd:element ref='cc' minOccurs='0' maxOccurs='unbounded'/>
           <xsd:element ref='aa' minOccurs='1' maxOccurs="1"/>
           <xsd:element ref='bb' minOccurs='1' maxOccurs="1"/>
       </xsd:choice>
   </xsd:complexType>
</xsd:element>
Unfortunately this wont work.
xmllint says that the above xml document is not valid.
Has anybody any idea concerning this problem? Is it generally possible 
to mix elements in a complexType  with different minOccurs/maxOccurs ?

Greetings, Peter 

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