XML Schema and the Unique Particle Attribution (UPA) rule

Hi,

I would like to create an XML Schema for the content model like:

<M>

  <type>type1</type>

  <… several elements specific to type1 …>

</M>

<M>

  <type>type2</type>

  <… several elements specific to type2 …>

</M>

Etc

I was playing with a XML Schema like:

  <xs:complexType name="upa_test">

    <xs:choice>

      <xs:sequence>

        <xs:element name="type" type="xs:string" fixed="type1" />

        <xs:element name="B" type="xs:string"/>

      </xs:sequence>

      <xs:sequence>

        <xs:element name="type" type="xs:string" fixed="type2" />

        <xs:element name="C" type="xs:string"/>

      </xs:sequence>

    </xs:choice>

  </xs:complexType>

But the processor I am using rejects this Schema saying it violates the UPA rule. Shouldn’t the processor be able to uniquely determine the particle based on the element’s type and/or value? Can someone suggest an alternate XML Schema that would not violate the UPA rule?

Changing the content model to introduce an intermediate level for <type1>, <type2>, etc would work, but I can’t make this change as it would break backward compatibility.

Thanks

Azariah