<xs:attribute name="freq" use="required">
    <xs:annotation>
        <xs:documentation>The FREQ rule part identifies the type of recurrence rule.</xs:documentation>
    </xs:annotation>
    <xs:simpleType>
        <xs:restriction base="xs:string">
            <xs:enumeration value="SECONDLY" />
            <xs:enumeration value="MINUTELY" />
            <xs:enumeration value="HOURLY" />
            <xs:enumeration value="DAILY" />
            <xs:enumeration value="WEEKLY" />
            <xs:enumeration value="MONTHLY" />
            <xs:enumeration value="YEARLY" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>