private void button1_Click(object sender, EventArgs e)
        {
            var list = new List<string>(){"1","2","666","3","4"};

            var rez = SplitBy("666", list);
        }

        IEnumerable<IEnumerable<string>> SplitBy(string sep, IEnumerable<string> seq)
        {
            List<string> currentPart = new List<string>();
            foreach (var value in seq)
            {
                if (EqualityComparer<string>.Default.Equals(value, sep))
                {
                    yield return currentPart;
                    currentPart = new List<string>();
                }
                else
                {
                    currentPart.Add(value);
                }
            }
            yield return currentPart;
        }