CurrentThreadScheduler allows cancellation of recursive or iterative observables without concurrency.

CurrentThreadScheduler

using System;
using System.Reactive.Concurrency;
using System.Reactive.Linq;

namespace ConsoleApplication1
{
  class Program
  {
    static void Main()
    {
      using (Observable.Range(0, 100, CurrentThreadScheduler.Instance)
                       .Do(x => Console.WriteLine("Generated: {0}", x))
                       .Take(10)
                       .Subscribe(x => Console.WriteLine("Observed: {0}", x)))
      {
        Console.ReadKey();
      }
    }
  }
}

Sorry, my bad. I understand the reason. Forget my question =)