I don't know if what I want to do is possible or not, but I'd like to have
an xs:key in my schema that constrains attributes both directly in a
document, as well as those in another document included in the first.
Here's an example of what I'm trying to say:

FooList.xsd: 
================ 
<?xml version="1.0" encoding="UTF-8"?> 
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
  xmlns:xi="http://www.w3.org/2001/XInclude" 
  xmlns:FooList="http://www.somecompany.com/FooList" 
  targetNamespace="http://www.somecompany.com/FooList" 
  xmlns="http://www.trionworld.com" elementFormDefault="qualified"> 
  <xs:import namespace="http://www.w3.org/2001/XInclude" 
    schemaLocation="http://www.w3.org/2001/XInclude.xsd"/> 
  <!--
    Range-limited data type for 'Bar' attributes on 'Foo's.  An xs:key will
    constrain the value to be unique (can you extend/restrict xs:ID to
    specify both min and max? ...).
  --> 
  <xs:simpleType name="BarAttr"> 
    <xs:restriction base="xs:nonNegativeInteger"> 
      <xs:minExclusive value="0"/> 
      <xs:maxInclusive value="65535"/> 
    </xs:restriction> 
  </xs:simpleType> 
  <xs:complexType name="Foo"> 
    <xs:attribute name="Bar" type="FooList:BarAttr" use="required"/> 
  </xs:complexType> 
  <xs:element name="FooList"> 
    <xs:complexType> 
      <xs:sequence minOccurs="0"> 
        <xs:element ref="xi:include" minOccurs="0" maxOccurs="unbounded"/> 
        <xs:element name="Foo" type="FooList:Foo" minOccurs="0"
maxOccurs="unbounded"/> 
      </xs:sequence> 
    </xs:complexType> 
    <xs:key name="PK_FooBar"> 
      <xs:selector xpath="FooList:Foo"/> 
      <xs:field xpath="@Bar"/> 
    </xs:key> 
  </xs:element> 
</xs:schema> 

FooListA.xml 
========== 
<?xml version="1.0" encoding="UTF-8"?> 
<FooList xmlns="http://www.somecompany.com/FooList" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xsi:schemaLocation="http://www.somecompany.com/FooList FooList.xsd"> 
  <Foo Bar="1"/> 
  <Foo Bar="2"/> 
</FooList> 

FooListB.xml 
========== 
<?xml version="1.0" encoding="UTF-8"?> 
<FooList xmlns="http://www.somecompany.com/FooList" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xsi:schemaLocation="http://www.somecompany.com/FooList FooList.xsd"> 
  <include xmlns="http://www.w3.org/2001/XInclude" href="FooListA.xml"
xpointer="FooList:FooList"/> 
  <Foo Bar="1"/> 
  <Foo Bar="2"/> 
</FooList> 


Essentially, I would like validation of FooListB.xml to fail, given that it
has 'Bar' attribute values which conflict with those in FooListA.xml
(assuming my include directive is correct...?).  I'm basically trying to
"merge" document fragments into a master document and have the key
constraint be useful.

Is this possible?
 
Cheers, 
-David