如何在xml架构中为递归类型定义Xpath选择器

嗨,

我有一个xml架构,其中包含以下类型：

<xs:complexType name="definition">
  <xs:sequence/>
  <xs:attribute name="id"/>
</xs:complexType>

<xsd:key name="definitionId">
  <xsd:selector xpath="definition"/>
  <xsd:field xpath="@id"/>
</xsd:key>

<xs:complexType name="elem">
  <xs:sequence>
    <xs:element name="entry1" type="elem" minOccurs="0" maxOccurs="unbounded"/>
    <xs:element name="entry2" type="xs:string" minOccurs="0" maxOccurs="unbounded"/>
  </xs:sequence>
  <xs:attribute name="ref" type="xs:string" use="required"/>
</xs:complexType>

这允许类似于：

<definition id="A">
<definition id="B">
<definition id="C">

<entry1 ref="A">
  <entry1 ref="B">
    <entry1 ref="C"/>
    <entry2/>
  </entry1>
  <entry1 ref="C">
  </entry1>
</entry1>

我需要一个XPath选择器来声明ref属性的keyref,但我不知道如何定义递归路径.

<xsd:keyref name="definitionRef" refer="definitionId">
  <xsd:selector xpath="???"/>  <<<how to find all "ref" of entry1 ?    
  <xsd:field xpath="@ref"/>
</xsd:keyref>

谢谢你的时间.