docs: private void _add(String fieldName, IonValueLite child)
      {
          hasNullFieldName |= fieldName == null;
          int size = get_child_count();
  
          // add this to the Container child collection
          add(size, child);
  
          // if we have a hash map we need to update it now
          if (_field_map != null) {
              add_field(fieldName, child._elementid());
          }
      }