code: def _update_subinstance(self, name, subform):
          
          old_subinstance = getattr(self.instance, name)
          new_subinstance = subform.update_instance()
  
          # if our instance previously had no node for the subform AND the
          # updated one has data, then attach the new node.
          if old_subinstance is None and not new_subinstance.is_empty():
              setattr(self.instance, name, new_subinstance)
  
          # on the other hand, if the instance previously had a node for the
          # subform AND the updated one is empty, then remove the node.
          if old_subinstance is not None and new_subinstance.is_empty():
              delattr(self.instance, name)
docs: Save bound data for a single subform into the XmlObject model
          instance.