RIGHTFOLD        1150 lj_opt_fold.c      return RIGHTFOLD;
RIGHTFOLD        1365 lj_opt_fold.c      return RIGHTFOLD;
RIGHTFOLD        1380 lj_opt_fold.c      return RIGHTFOLD;
RIGHTFOLD        1637 lj_opt_fold.c      return RIGHTFOLD;  /* (b o1 a) o2 b ==> b; (a o1 b) o2 b ==> b */