加强型无穷集合：InfiniteList<T>

　　 //有个 Step 属性用来指定遍历方向和偏移量，遍历期间被设置为 0 就终止遍历。
    public class InfiniteList<T> : IEnumerable<T> {
        public List<T> Source { get; }
        int start { get; }
        public int Step { get; set; } = 1;

        public InfiniteList(IEnumerable<T> source) : this(0, source) {
        }

        public InfiniteList(int start, IEnumerable<T> source) {
            this.start = Math.Max(Math.Min(start, source.Count()), -1);
            Source = new List<T>(source);
        }

        public IEnumerator<T> GetEnumerator() {
            return new Enumerator(this);
        }

        IEnumerator IEnumerable.GetEnumerator() {
            return GetEnumerator();
        }

        class Enumerator : IEnumerator<T> {
            InfiniteList<T> list;

            int index = -1;
            public Enumerator(InfiniteList<T> source) {
                this.list = source;
                index = source.start - source.Step;
            }

            public T Current {
                get { return list.Source[index]; }
            }

            object IEnumerator.Current {
                get { return Current; }
            }

            public void Dispose() {
            }

            public bool MoveNext() {
                if (list.Source.Count == 0) {
                    return false;
                }
                if (list.Step == 0) {
                    return false;
                }
                index += list.Step;
                while (index > list.Source.Count - 1) {
                    index -= list.Source.Count;
                }
                while (index < 0) {
                    index += list.Source.Count;
                }

                return true;
            }

            public void Reset() {
                index = list.start;
            }
        }
    }