1  <?xml version="1.0" encoding="utf-8"?>
 2  <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 3    <xs:element name="odd-page" type="xs:string"/>
 4    <xs:element name="even-page" type="xs:string"/>
 5    <xs:element name="book">
 6      <xs:complexType>
 7        <xs:sequence>
 8          <xs:sequence minOccurs="0" maxOccurs="unbounded">
 9            <xs:element ref="odd-page"/>
10            <xs:element ref="even-page"/>
11          </xs:sequence>
12          <xs:element ref="odd-page" minOccurs="0"/>
13        </xs:sequence>
14      </xs:complexType>
15    </xs:element>
16  </xs:schema>