How do I refer to another property in a jnlp?

Question

<property name="netbeans.user" value="${user.home}/.foo"/>
        <property name="derby.system.home" value="netbeans.user">

I want derby.system.home property to be the same as the above one. How do I achieve that other than by expliciting the same CDATA (so by value)?

I guess what I'm asking is: how can I let the value of an element attribute but the same of another element, by reference?

PS: I'm trying with this jnlp.