[Test] public void AddIfAbsentDoesAddWhenNotExist()
        {
            var sut = new CopyOnWriteList <T>(Samples);

            sut.AddIfAbsent(TestData <T> .M1);
            Assert.IsTrue(sut.Contains(TestData <T> .M1));
        }
        [Test] public void AddIfAbsentDoesNotAddWhenExist()
        {
            var sut = new CopyOnWriteList <T>(Samples);

            sut.AddIfAbsent(TestData <T> .One);
            Assert.That(sut.Count, Is.EqualTo(SampleSize));
        }