private ObservableMap<URI, Object> getHeapIdMap() {
   if (_HeapIdMap == null) {
     _HeapIdMap = Observables.observableMap(new HashMap<URI, Object>());
   }
   return _HeapIdMap;
 };
 private ObservableMap<Integer, Model> getHashCodeMap() {
   if (_HashCodeMap == null) {
     _HashCodeMap = Observables.observableMap(new HashMap<Integer, Model>());
   }
   return _HashCodeMap;
 }