Need something like xs:sequence without order?

Hi experts,

I have some tricky XML that needs to be validated against an XSD, that I am
not sure how to define.
The XML could come in several variations. Some examples:

Example 1:

<SomeElement>
  <A>aaa</A>
  <B>bbb</B>
  <C>ccc</C>
  <D>ddd</D>
  <D>dde</D>
</SomeElement>

Example 2:

<SomeElement>
  <B>bbb</B>
  <A>aaa</A>
  <C>ccc</C>
  <D>ddd</D>
  <D>dde</D>
</SomeElement>

Example 3:

<SomeElement>
  <B>bbb</B>
  <A>aaa</A>
  <D>ddd</D>
  <D>dde</D>
</SomeElement>


The corresponding XSD that I would like to define (but of course is invalid)
would look like this:

<xs:element name="SomeElement">
  <xs:complexType>
    <xs:all>
     <xs:element name="A" type="xs:string" minOccurs="1" maxOccurs="1" />
     <xs:element name="B" type="xs:string" minOccurs="1" maxOccurs="1" />
     <xs:element name="C" type="xs:string" minOccurs="0" maxOccurs="1" />

     <xs:element name="D" type="xs:string" minOccurs="1"
maxOccurs="unbounded" />

    </xs:all>
  </xs:complexType>
</xs:element>

Any ideas how to achive the same with valid XSD?
Thanks in advance,

Felix