code: def add_child(self, child):
          
          child.set_model(self._model)
          if self._model:
              row = len(self.childItems)
              parentindex = self._model.index_of_item(self)
              self._model.insertRow(row, child, parentindex)
          else:
              self.childItems.append(child)
docs: Add child to children of this TreeItem
  
          :param child: the child TreeItem
          :type child: :class:`TreeItem`
          :returns: None
          :rtype: None
          :raises: None