CrossDualitySelect.cs

        static void Main(string[] args)
        {
            var items = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }
                .Select(x =>
                {
                    Console.WriteLine(x);
                    return x;
                })
                .Select(x => Observable.Range(x, 1))
                .Concat()
                .Take(3)
                .Subscribe(x =>
                {
                    Console.WriteLine("subscribe: {0}", x);
                });

            Console.ReadLine();
        }