Re: constraints on model group schema components: Unique Particle Att ribution

In your example, the fragment:

<xs:element name = "child">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref = "x" minOccurs = "0"></xs:element>
        <xs:element ref = "x" minOccurs = "0"></xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

violates the unique article attribution constraint.  Reason: when 
validating the instance

<child>
        <x/>
</child>

you don't know whether the <x/> matches the first for the second reference 
to x.    If you look at the specification at [1] you will see that the 
declaration of the sequence turns into a list of two particles.  The fact 
that both particles reference the same declaration as their {term} [2] 
does not result in the two particles being combined.  There are two 
particles and thus an ambiguity.

[1] http://www.w3.org/TR/xmlschema-1/#Model_Group_details
[2] http://www.w3.org/TR/xmlschema-1/#cParticles
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Noah Mendelsohn                              Voice: 1-617-693-4036
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