<xsd:element name="changefreq">
    <xsd:annotation>
        <xsd:documentation>
      OPTIONAL: Indicates how frequently the content at a particular URL is
      likely to change. The value "always" should be used to describe
      documents that change each time they are accessed. The value "never"
      should be used to describe archived URLs. Please note that web
      crawlers may not necessarily crawl pages marked "always" more often.
      Consider this element as a friendly suggestion and not a command.
    </xsd:documentation>
    </xsd:annotation>
    <xsd:simpleType>
        <xsd:restriction base="xsd:string">
            <xsd:enumeration value="always" />
            <xsd:enumeration value="hourly" />
            <xsd:enumeration value="daily" />
            <xsd:enumeration value="weekly" />
            <xsd:enumeration value="monthly" />
            <xsd:enumeration value="yearly" />
            <xsd:enumeration value="never" />
        </xsd:restriction>
    </xsd:simpleType>
</xsd:element>