def add_set(self):
self.__label_counter += <NUM_LIT:1><EOL>new_label = self.__label_counter<EOL>self.__forest[new_label] = -<NUM_LIT:1>  <EOL>self.__set_counter += <NUM_LIT:1><EOL>return new_label<EOL>
Adds a new set to the forest.
        Returns a label by which the new set can be referenced