code: def all_possible_actions(self) -> List[str]:
          
          all_actions = set()
          for _, action_list in self.valid_actions.items():
              for action in action_list:
                  all_actions.add(action)
          return sorted(all_actions)
docs: Return a sorted list of strings representing all possible actions
          of the form: nonterminal -> [right_hand_side]