public void AddTest()
{
int?x = 1;
int?y = 1;
int?z = null;
Assert.That(PatternMatching.Add(x, y), Is.EqualTo(2));
Assert.That(PatternMatching.Add(x, z), Is.EqualTo(1));
Assert.That(PatternMatching.Add(z, y), Is.EqualTo(1));
Assert.That(PatternMatching.Add(z, z), Is.EqualTo(0));
}