docs: def get_label_set(self, type_str=None):
          """Get a set of label_str for the tree rooted at this node.
  
          Args:
              type_str:
                  SUBJECT_NODE_TAG, TYPE_NODE_TAG or None. If set, only include
                  information from nodes of that type.
  
          Returns:
              set: The labels of the nodes leading up to this node from the root.
  
          """
          return {v.label_str for v in self.node_gen if type_str in (None, v.type_str)}