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

            sut.AddRangeAbsent(new T[0]);
            Assert.That(sut.Count, Is.EqualTo(SampleSize));
        }
        [Test] public void AddRangeIfAbsentAddOnlyThoseNotExist()
        {
            var sut = new CopyOnWriteList <T>(Samples);
            var c   = new[]
            {
                TestData <T> .M1,
                TestData <T> .M2,
                TestData <T> .One,           // Will no add.
            };

            Assert.That(sut.AddRangeAbsent(c), Is.EqualTo(2));
            Assert.That(sut.Count, Is.EqualTo(SampleSize + 2));
            Assert.IsTrue(sut.Contains(TestData <T> .M1));
            Assert.IsTrue(sut.Contains(TestData <T> .M2));
        }