nn_mod_pow_redc
ATTRIBUTE_WARN_UNUSED_RET int nn_mod_pow_redc(nn_t out, nn_src_t base, nn_src_t exp, nn_src_t mod, nn_src_t r, nn_src_t r_square, word_t mpinv);
return nn_mod_pow_redc(&(out->fp_val), &(base->fp_val), exp, &(out->ctx->p), &(out->ctx->r), &(out->ctx->r_square), out->ctx->mpinv);
ret = nn_mod_pow_redc(out, base, exp, mod, &r, &r_square, mpinv);
ret = nn_mod_pow_redc(out, x, &p_minus_two, p, r, r_square, mpinv);