I have a subclass element, which happens to be a mix of table per class hierarchy with table per subclass.  My question is, why am I not allowed to use the <set> mapping within the <join> element?  What is the alternative?
Code:
<subclass name="bean.Employer" discriminator-value="E">
            <join table="EMPLOYER">
                <key column="USER_ID"/>
                <property name="name" column="NAME" type="java.lang.String"/>
                <property name="description" column="DESCRIPTION" type="java.lang.String"/>
                <set name="locations">
                    <key>
                       <column name="EMP_ID" precision="12" scale="0"/>
                    </key>
                    <one-to-many class="bean.EmpLocation"/>
                 </set>
            </join>
        </subclass>