nn_xgcd
ATTRIBUTE_WARN_UNUSED_RET int nn_xgcd(nn_t g, nn_t u, nn_t v, nn_src_t a, nn_src_t b, int *sign);
GENERIC_TEST_NN(nn_xgcd, NN_XGCD, "xgcd", nn_xgcd, "nnnnns", "oooiio",
ret = nn_xgcd(g, &u, &v, a, b, sign);
ret = nn_xgcd(&out, &u, &v, x, m, &sign); EG(ret, err);