N_TSHIFT
#define INCREF(x) ((((x)&~N_BTMASK)<<N_TSHIFT)|(DT_PTR<<N_BTSHFT)|(x&N_BTMASK))
#define DECREF(x) ((((x)>>N_TSHIFT)&~N_BTMASK)|((x)&N_BTMASK))
