<xs:group name="elementsGroup">
    <xs:sequence>
        <xs:choice minOccurs="0" maxOccurs="unbounded">
            <xs:element ref="title" />
            <xs:element ref="creator" />
            <xs:element ref="subject" />
            <xs:element ref="description" />
            <xs:element ref="publisher" />
            <xs:element ref="contributor" />
            <xs:element ref="date" />
            <xs:element ref="type" />
            <xs:element ref="format" />
            <xs:element ref="identifier" />
            <xs:element ref="source" />
            <xs:element ref="language" />
            <xs:element ref="relation" />
            <xs:element ref="coverage" />
            <xs:element ref="rights" />
        </xs:choice>
    </xs:sequence>
</xs:group>