tuple_flags
#define LINK_SPACE(f) (((struct tuple_flags *)(&(f)))->link_space)
#define HAS_LINK(f) (((struct tuple_flags *)(&(f)))->has_link)
#define MFC_FN(f) (((struct tuple_flags *)(&(f)))->mfc_fn)
#define SPACE(f) (((struct tuple_flags *)(&(f)))->space)