private void CheckDeterminantDD(double x1, double y1, double x2, double y2, double expected, double errBound)
        {
            var det = DD.Determinant(
                DD.ValueOf(x1), DD.ValueOf(y1),
                DD.ValueOf(x2), DD.ValueOf(y2));

            CheckErrorBound("Determinant", det, DD.ValueOf(expected), errBound);
        }