docs: @SuppressWarnings("WeakerAccess")
      public static synchronized SlotReference getSlotReference(int player, CdjStatus.TrackSourceSlot slot) {
          Map<CdjStatus.TrackSourceSlot, SlotReference> playerMap = instances.get(player);
          if (playerMap == null) {
              playerMap = new HashMap<CdjStatus.TrackSourceSlot, SlotReference>();
              instances.put(player, playerMap);
          }
          SlotReference result = playerMap.get(slot);
          if (result == null) {
              result = new SlotReference(player, slot);
              playerMap.put(slot, result);
          }
          return result;
      }