code: def add_child(self, widget):
          
          li_itm = _li(id=self.id + str(self._count))
          li_itm.add_child(widget)
  
          super(List, self).add_child(li_itm)
          self._items.append((widget, li_itm))
          self._count += 1
docs: Append a widget to the list.
  
          :param widget: Object inheriting :class:`~.widgets.base.BaseElement`