<grammar xmlns=“relaxng.org/ns/structure/1.0”>

<start>
  <element name="addressBook">
    <zeroOrMore>
      <element name="card">
        <ref name="card.attlist"/>
      </element>
    </zeroOrMore>
  </element>
</start>

<define name="card.attlist">
  <group>
    <attribute name="name">
      <text/>
    </attribute>
    <attribute name="email">
      <text/>
    </attribute>
  </group>
</define>

</grammar>