void TestSubtraction (Vector4 a, Vector4 b, Vector4 expected )
{
// This test asserts the following:
// a - b == expected
// b - a == -expected
var result_1a = a - b;
var result_2a = b - a;
Vector4 result_1b; Vector4.Subtract(ref a, ref b, out result_1b);
Vector4 result_2b; Vector4.Subtract(ref b, ref a, out result_2b);
Assert.That(result_1a, Is.EqualTo(expected));
Assert.That(result_2a, Is.EqualTo(-expected));
Assert.That(result_1b, Is.EqualTo(expected));
Assert.That(result_2b, Is.EqualTo(-expected));
}