code: public static <CustomSubjectBuilderT extends CustomSubjectBuilder>
        CustomSubjectBuilderT assertAbout(
            CustomSubjectBuilder.Factory<CustomSubjectBuilderT> factory) {
      return assert_().about(factory);
    }
docs: A generic, advanced method of extension of Truth to new types, which is documented on {@link
  CustomSubjectBuilder}. Extension creators should prefer {@link Subject.Factory} if possible.