另外，小弟对引用一直搞得不是太清楚，请帮忙解释一下引用的用法，谢谢

<schema xmlns='http://www.w3.org/1999/XMLSchema'
        targetNamespace='someURI'
        xmlns:tns='someURI'>
  <group [color=#FF0000]name='c1'[/color]>
    <sequence>
      <element name='color' type='string' />
      <element name='length' type='string' />
    </sequence>
  </group>
  <group [color=#FF0000]name='cc1'[/color]>
    <choice>
      <element name='cap' type='string' />
      <group ref='tns:c1' maxOccurs='unbounded' />
    </choice>
  </group>
  <complexType name='Headpiece' content='elementOnly'>
    <group ref='tns:cc1' minOccurs='0' />
  </complexType>
  <element name='hp' type='tns:Headpiece' />
</schema>

用法：印用只能对全局元素有效