<xs:schema xmlns:xs=“www.w3.org/2001/XMLSchema”>

<xs:element name="odd-page" type="xs:string"/>
<xs:element name="even-page" type="xs:string"/>
<xs:element name="book">
  <xs:complexType>
    <xs:sequence>
      <xs:sequence minOccurs="0" maxOccurs="unbounded">
        <xs:element ref="odd-page"/>
        <xs:element ref="even-page"/>
      </xs:sequence>
      <xs:element ref="odd-page" minOccurs="0"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

</xs:schema>