LIMB_MASK
a->limb[LIMBPERM(i)] = scarry & LIMB_MASK(LIMBPERM(i));
a->limb[LIMBPERM(i)] = carry & LIMB_MASK(LIMBPERM(i));
x->limb[LIMBPERM(i)] = (word_t)((i < NLIMBS - 1) ? buffer & LIMB_MASK(LIMBPERM(i)) : buffer);