<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:group name="name">

    <xs:choice>

      <xs:element name="name" type="xs:string"/>

      <xs:sequence>

        <xs:element name="first-name" type="xs:string"/>

        <xs:element name="middle-name" type="xs:string" minOccurs="0"/>

        <xs:element name="last-name" type="xs:string"/>

      </xs:sequence>

    </xs:choice>

  </xs:group>

  <xs:element name="author">

    <xs:complexType>

      <xs:group ref="name"/>

    </xs:complexType>

  </xs:element>

</xs:schema>