ences in <code>ref</code> are atomically cleared.
   </li>
   <li>
     All objects formerly being referenced by <code>ref</code> become eligible for
     finalization.
   </li>
   <li>
     At some future point, all references in <code>ref</code> will be enqueued
     with their corresponding reference queues, if any.
   </li>
 </ul>

 Weak references are useful for mappings that should have their entries
 removed automatically once they are not referenced any more (from outside).
 The difference between a