What the below fields are meant for?
hit_next_index ( https://docs.fd.io/vpp/17.04/df/de9/structvl__api__classify__add__del__session__t.html#a84e237caa7d977236be50cbae42ef7c1 ) ; opaque_index ( https://docs.fd.io/vpp/17.04/df/de9/structvl__api__classify__add__del__session__t.html#a9d2cc4ffd3b8f0fc7cd57d0dae8c37c8 ) ; advance ( https://docs.fd.io/vpp/17.04/df/de9/structvl__api__classify__add__del__session__t.html#a4cdecc5b93f473fe36ee880a436e2f77 ) ;