nn_gcd
ATTRIBUTE_WARN_UNUSED_RET int nn_gcd(nn_t d, nn_src_t a, nn_src_t b, int *sign);
GENERIC_TEST_NN(nn_gcd, NN_GCD, "gcd", nn_gcd, "nnns", "oiio",
ret = nn_gcd(d, &tmp, n, &sign); EG(ret, err);