Adds a new set to the forest.
        Returns a label by which the new set can be referenced

def add_set(self):
        """Adds a new set to the forest.
        Returns a label by which the new set can be referenced
        """
        self.__label_counter += 1
        new_label = self.__label_counter
        self.__forest[new_label] = -1  # All new sets have their parent set to themselves
        self.__set_counter += 1
        return new_label