text: class DuelCenter:
      def __init__(self, players):
          if len(players) != 2:
              raise ValueError()
  
          self._current_player, self._enemy_player = players
  
      def _swap_player(self):
          self._current_player, self._enemy_player = (self._enemy_player, self._current_player)
  
      def _round(self):
          self._current_player.attack(self._enemy_player)
          self._swap_player()
  
      def start_duel(self):
          while self._current_player.is_alive() and self._enemy_player.is_alive():
              self._round()
  