docs: public Object setProperty( Property propertyId,
                                 Object value ) {
          if (value == null) {
              // Removing this property ...
              return nodeProperties != null ? nodeProperties.remove(propertyId) : null;
          }
          // Otherwise, we're adding the property
          if (nodeProperties == null) nodeProperties = new TreeMap<Property, Object>();
          return nodeProperties.put(propertyId, value);
      }