<xsd:element name="reference">
    <xsd:annotation>
        <xsd:documentation>This element links the OVAL Definition to a definitive external reference.  For example, CVE Identifiers for vulnerabilities.  The intended purpose for this reference is to link the definition to a variety of other sources that share this common name or identifier.  Only one reference is allowed for each definition and the required source attribute serves as an indicator to the reference type.</xsd:documentation>
        <xsd:appinfo>
            <cardinality>0-1</cardinality>
            <attributes>source</attributes>
            <content>string</content>
            <parent_elements>definition</parent_elements>
            <child_elements>none</child_elements>
        </xsd:appinfo>
    </xsd:annotation>
    <xsd:complexType>
        <xsd:simpleContent>
            <xsd:extension base="xsd:anyURI">
                <xsd:attribute name="source" type="oval_results:reference_source" use="required" />
            </xsd:extension>
        </xsd:simpleContent>
    </xsd:complexType>
</xsd:element>