private NullsBeforeB() {
      super("b");
    }
  }

  public static final class NullsBeforeTwo extends NullsBefore {
    public static final NullsBeforeTwo INSTANCE = new NullsBeforeTwo();

    private NullsBeforeTwo() {
      super("two"); // from TestStringSortedMapGenerator's sample keys
    }
  }

  @J2ktIncompatible
  @GwtIncompatible // reflection