code: public static void registerSerializers(final Kryo kryo) {
  
      // ImmutableSortedSet (abstract class)
      //  +- EmptyImmutableSortedSet
      //  +- RegularImmutableSortedSet
      //  +- DescendingImmutableSortedSet
  
      final ImmutableSortedSetSerializer serializer = new ImmutableSortedSetSerializer();
  
      kryo.register(ImmutableSortedSet.class, serializer);
      kryo.register(ImmutableSortedSet.of().getClass(), serializer);
      kryo.register(ImmutableSortedSet.of("").getClass(), serializer);
      kryo.register(ImmutableSortedSet.of().descendingSet ().getClass(), serializer);
    }
docs: Creates a new {@link ImmutableSortedSetSerializer} and registers its serializer
  for the several ImmutableSortedSet related classes.
  
  @param kryo the {@link Kryo} instance to set the serializer on