public void CanEnumerateListWhileSomethingChangesIt()
        {
            var list = new SlowImmutableList<int>(new[] { 1, 2, 3, 4 });
            var enumerator = list.GetEnumerator();
            enumerator.MoveNext().ShouldBe(true);

            var list2 = list.Add(1);
            Should.NotThrow(() => enumerator.MoveNext().ShouldBe(true));
        }