in one transaction. Garbage collection can be used together with
explicit deallocator by <code>Persistent.deallocate</code> method. But when you are using garbage collector, 
you should be careful with keeping references to the persistent objects in local variables. If there are no references
to such object from other persistent object (so it is not reachable from root object), then garbage collector
can deallocate such object. If you then try to access or update this object using reference stored 
in local