code: public static void registerSerializers(final Kryo kryo) {
  
          // UnmodifiableNavigableSetSerializer (private class)
  
          final UnmodifiableNavigableSetSerializer serializer = new UnmodifiableNavigableSetSerializer();
  
          kryo.register(Sets.unmodifiableNavigableSet(new TreeSet<Object>()).getClass(), serializer);
      }
docs: Creates a new {@link UnmodifiableNavigableSetSerializer} and registers its serializer
  for the UnmodifiableNavigableSetSerializer related class.
  
  @param kryo the {@link Kryo} instance to set the serializer on