tag2tt
#define NILCONSTANT {{{NULL}, tag2tt(LUA_TNIL)}}
#define NILCONSTANT {{tag2tt(LUA_TNIL), {NULL}}}
#define settt_(o,t) (tt_(o) = tag2tt(t))
#define checktag(o,t) (tt_(o) == tag2tt(t))
#define checktype(o,t) (ctb(tt_(o) | VARBITS) == ctb(tag2tt(t) | VARBITS))