<code>store()</code> method in this object <B>X</B>. But then if you update one of the elements in this tree, 
you should invoke <code>store()</code> individually for each such element (<code>X.store()</code> will
NOT now automatically save referenced objects).<P>

<code>Persistent.modify</code> method mark object is modified but doesn't immediately write it to the storage.
All objects marked as modified will be stored to the storage during transaction commit (<code>store</code> method
will be invoked for e