code: def get_parent_label(self, treepos):
          
          parent_pos = self.get_parent_treepos(treepos)
          if parent_pos is not None:
              parent = self.dgtree[parent_pos]
              return parent.label()
          else:
              return None
docs: Given the treeposition of a node, return the label of its parent.
          Returns None, if the tree has no parent.