MP_HOWMANY
#define MP_ROUNDUP(a,b) (MP_HOWMANY(a,b) * (b))
ix = MP_HOWMANY(k, MP_DIGIT_BIT) + MP_USED(p) + 1;
ARGCHK(MP_HOWMANY(lsbNum, MP_DIGIT_BIT) <= MP_USED(a), MP_RANGE);