Permutations extension

EnumerableExtensions.cs

  public static class EnumerableExtensions
    {

        public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> source)
        {
            if (source == null)
                throw new ArgumentNullException("source");
            // Ensure that the source IEnumerable is evaluated only once
            return permutations(source.ToArray());
        }

        private static IEnumerable<IEnumerable<T>> permutations<T>(IEnumerable<T> source)
        {
            var c = source.Count();
            if (c == 1)
                yield return source;
            else
                for (int i = 0; i < c; i++)
                    foreach (var p in permutations(source.Take(i).Concat(source.Skip(i + 1))))
                        yield return source.Skip(i).Take(1).Concat(p);
        }
    }

program.cs

    public class Program
    {
        public static void Main()
        {
            List<string> list = new List<string> { "ОТ", "ТРПО", "НПО", "ЗН", "ПОВС" };

            var perm = list.Permutations<string>().Where(enumerable => enumerable.Last() == "ПОВС").Where(enumerable => enumerable.First() == "ОТ");
            foreach (var l in perm)
            {
                string s = string.Join(", ", l);
                Console.WriteLine(s);
            }
        }
    }