docs: public static synchronized DeckReference getDeckReference(int player, int hotCue) {
          Map<Integer, DeckReference> playerMap = instances.get(player);
          if (playerMap == null) {
              playerMap = new HashMap<Integer, DeckReference>();
              instances.put(player, playerMap);
          }
          DeckReference result = playerMap.get(hotCue);
          if (result == null) {
              result = new DeckReference(player, hotCue);
              playerMap.put(hotCue, result);
          }
          return result;
      }