code: @Override
      public void addListener(IChemObjectListener col) {
          List<IChemObjectListener> listeners = lazyChemObjectListeners();
  
          if (!listeners.contains(col)) {
              listeners.add(col);
          }
          // Should we throw an exception if col is already in here or
          // just silently ignore it?
      }
docs: Use this to add yourself to this IChemObject as a listener. In order to do
  so, you must implement the ChemObjectListener Interface.
  
  @param  col  the ChemObjectListener
  @see         #removeListener