I have the following schema:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
    targetNamespace="http://www.ubs.com/namespace/wbml-render"
    xmlns:wbr="http://www.ubs.com/namespace/wbml-render"
    xmlns="http://www.ubs.com/namespace/wbml-render"
elementFormDefault="qualified"
    attributeFormDefault="unqualified">
    <xs:complexType name="footnoteExplanation">
        <xs:annotation>
            <xs:documentation>The text explaining the
footnote.</xs:documentation>
        </xs:annotation>
        <xs:simpleContent>
            <xs:extension base="xs:string"/>
        </xs:simpleContent>
    </xs:complexType>
    <xs:complexType name="footnote">
        <xs:annotation>
            <xs:documentation>Composed of base and
explanation.</xs:documentation>
        </xs:annotation>
        <xs:sequence>
            <xs:element name="footnoteBase" type="footnoteBase"/>
            <xs:element name="footnoteExplanation"
type="footnoteExplanation" minOccurs="0"/>
        </xs:sequence>
        <xs:attribute name="footnoteId" type="xs:NMTOKEN"
use="optional"/>
        <xs:attribute name="footnoteRef" type="xs:NMTOKEN"
use="optional"/>
    </xs:complexType>
    <xs:complexType name="footnoteBase" mixed="true">
        <xs:annotation>
            <xs:documentation>The text to which the footnote
pertains.</xs:documentation>
        </xs:annotation>
        <xs:sequence>
            <xs:element ref="footnote" minOccurs="0"
maxOccurs="unbounded"/>
        </xs:sequence>       
    </xs:complexType>
    <xs:element name="footnote" type="footnote">
        <!--<xs:key name="ref_or_explanation">
            <xs:selector xpath="."/>
            <xs:field xpath="wbr:footnoteExplanation | @footnoteRef"/>
        </xs:key>-->
    </xs:element>
    <xs:element name="para">
        <xs:complexType mixed="true">
            <xs:sequence>
                <xs:element ref="footnote" minOccurs="0"
maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
        <xs:key name="ref_or_explanation">
            <xs:selector xpath=".//wbr:footnote"/>
            <xs:field xpath="wbr:footnoteExplanation | @footnoteRef"/>
        </xs:key>
    </xs:element>
</xs:schema>

and the following XML schema instance

<?xml version="1.0" encoding="UTF-8"?>
<wbr:para xmlns:wbr="http://www.ubs.com/namespace/wbml-render"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.ubs.com/namespace/wbml-render
 
file:/Users/richard/Documents/Work/Accessibility/Schemas/test_footnote.x
sd"> Now is 
    <wbr:footnote footnoteRef="abc">
        <wbr:footnoteBase>the</wbr:footnoteBase>
        <!--<wbr:footnoteExplanation>As Clinton said, "It depends on
what you mean by 'the'."</wbr:footnoteExplanation>-->
    </wbr:footnote> for 
    <wbr:footnote>
        <wbr:footnoteBase>
            <wbr:footnote>
                <wbr:footnoteBase>
                    <wbr:footnote footnoteRef="x22">
                        <wbr:footnoteBase>all</wbr:footnoteBase>
                    </wbr:footnote> good
                </wbr:footnoteBase>
                <wbr:footnoteExplanation>In some inner
sense.</wbr:footnoteExplanation>
            </wbr:footnote> men
        </wbr:footnoteBase>
        <wbr:footnoteExplanation>Good women,
too.</wbr:footnoteExplanation>
    </wbr:footnote>
</wbr:para>

The xs:key on the para element is supposed to ensure that each footnote
descendant has either a footnoteExplanation child or a footnoteRef
attribute.  The first footnote child of para "works" as desired (in
Oxygen 7, i.e., Xerces):  Uncommenting the footnoteExplanation element
results in an error, as does deleting the footnoteRef attribute.

The second footnote element is rather complicated, because the model
permits "nesting" footnotes.  The XML seems valid:  The second footnote
element has a footnoteExplanation and no footnoteRef, so does its first
footnote descendent, and the innermost footnote has a footnoteRef
attribute and no footnoteExplanation.

Nevertheless, I receive the following two error messages (I've pasted
the offending statement after the Location):

SystemID:
/Users/richard/Documents/Work/Accessibility/Schemas/test_footnote.xml
Location: 18:88
                <wbr:footnoteExplanation>In some inner
sense.</wbr:footnoteExplanation>
Description: E Identity constraint error:  field
"./wbr:footnoteExplanation|@footnoteRef" matches more than one value
within the scope of its selector; fields must match unique values.

SystemID:
/Users/richard/Documents/Work/Accessibility/Schemas/test_footnote.xml
Location: 21:76
        <wbr:footnoteExplanation>Good women,
too.</wbr:footnoteExplanation>
Description: E Identity constraint error:  field
"./wbr:footnoteExplanation|@footnoteRef" matches more than one value
within the scope of its selector; fields must match unique values.

Can anybody tell me what I'm missing?

Thanks,
Richard

From george@o... Thu Feb 16 09:49:08 2006
Received: from maggie.w3.org ([193.51.208.68])
	by frink.w3.org with esmtp (Exim 4.50)
	id 1F9fl6-0004m8-I8
	for xmlschema-