code: private void _idAdded(String clientId)
    {
      // An id to visit has been added, update our other
      // collections to reflect this.
  
      // Update the ids collection
      _ids.add(_getIdFromClientId(clientId));
  
      // Update the unvisited ids collection
      _unvisitedClientIds.add(clientId);
  
      // Update the subtree ids collection
      _addSubtreeClientId(clientId);
    }
docs: an new id has been added.