code: def _uncheck_ancestor(self, item):
          
          self.change_state(item, "unchecked")
          parent = self.parent(item)
          if parent:
              children = self.get_children(parent)
              b = ["unchecked" in self.item(c, "tags") for c in children]
              if False in b:
                  # at least one box is checked and item's box is unchecked
                  self._tristate_parent(parent)
              else:
                  # no box is checked
                  self._uncheck_ancestor(parent)
docs: Uncheck the box of item and change the state of the boxes of item's
          ancestors accordingly.