<xs:attribute name="turn" use="optional">
    <xs:simpleType id="turn">
        <xs:restriction base="xs:string">
            <xs:enumeration id="turnInturn" value="inturn">
                <xs:annotation>
                    <xs:documentation>When the stone is released with a clock-wise
                                turn.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration id="turnOutturn" value="outturn">
                <xs:annotation>
                    <xs:documentation>When the stone is released with a anti-clock-wise
                                turn.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>