<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:tn="foo"
           targetNamespace="foo" elementFormDefault="qualified">
    <xs:element name="root">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="a">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element ref="tn:b"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
                <xs:element name="ref" maxOccurs="unbounded" minOccurs="0" type="xs:string" />
            </xs:sequence>
        </xs:complexType>
                                        
        <!-- identity constraint -->
        <xs:key name="key">
            <xs:selector xpath=".//tn:a/tn:b|.//tn:a/tn:b/tn:c"/>
            <xs:field xpath="@id"/>
        </xs:key>
        <xs:keyref name="keyref" refer="tn:key">
            <xs:selector xpath=".//tn:ref"/>
            <xs:field xpath="."/>
        </xs:keyref>
    </xs:element>
    
    <xs:element name="c">
        <xs:complexType>
            <xs:attribute name="id" type="xs:string"/>
        </xs:complexType>
    </xs:element>
                
    <xs:element name="b">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="tn:c" minOccurs="0" />
            </xs:sequence>
            <xs:attribute name="id" type="xs:string"/>
        </xs:complexType>
    </xs:element>
</xs:schema>