s_mp_invmod_radix
mmm->n0prime = 0 - s_mp_invmod_radix(MP_DIGIT(&meth->irr, 0));
mp_digit s_mp_invmod_radix(mp_digit P); /* returns (P ** -1) mod RADIX */
r = 0 - s_mp_invmod_radix(MP_DIGIT(p,0));
mp_digit i = s_mp_invmod_radix(MP_DIGIT(a,0));