<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="library">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="book" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="isbn" type="xs:string"/>
              <xs:element name="title" type="xs:string"/>
              <xs:element name="author-ref" minOccurs="0" maxOccurs="unbounded">
                <xs:complexType>
                  <xs:attribute name="id" type="xs:IDREF" use="required"/>
                </xs:complexType>
              </xs:element>
              <xs:element name="character-ref" minOccurs="0" maxOccurs="unbounded">
                <xs:complexType>
                  <xs:attribute name="id" type="xs:IDREF" use="required"/>
                </xs:complexType>
              </xs:element>
            </xs:sequence>
            <xs:attribute name="id" type="xs:ID" use="required"/>
          </xs:complexType>
        </xs:element>
        <xs:element name="author" minOccurs="0" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element ref="name"/>
              <xs:element name="nickName" type="xs:string"/>
              <xs:element name="born" type="xs:string"/>
              <xs:element name="dead" type="xs:string"/>
            </xs:sequence>
            <xs:attribute name="id" type="xs:ID" use="required"/>
          </xs:complexType>
        </xs:element>
        <xs:element name="character" minOccurs="0" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence>
              <xs:element ref="name"/>
              <xs:element name="since" type="xs:string"/>
              <xs:element name="qualification" type="xs:string"/>
            </xs:sequence>
            <xs:attribute name="id" type="xs:ID" use="required"/>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="name" type="xs:string"/>
</xs:schema>

