Properties do not have to derive from subproperties. They can also derive from properties in the same item. For example, suppose a user
item descriptor defines a home address and a shipping address. The ship-to address can inherit its value from the home address, like this:
<item-descriptor name="user"> <property name="shipToAddress" writable="false" item-type="address"> <derivation> <expression>shippingAddress</expression> <expression>homeAddress</expression> </derivation> </property> <table name="user" > <property name="shippingAddress" item-type="address"> <property name="homeAddress" item-type="address"> </table> </item-descriptor>