public void TestArrayAdd() {
var array = new[] { 1, 2, 3 };
array = array.Add(4);
Assert.AreEqual(4, array.Length);
Assert.AreEqual(1, array[0]);
Assert.AreEqual(2, array[1]);
Assert.AreEqual(3, array[2]);
Assert.AreEqual(4, array[3]);
}