Apply "yield return" recursively - iterating tree data structures

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace TreeIterations {
   static class Program {
      /// <summary>
      /// Der Haupteinstiegspunkt f�r die Anwendung.
      /// </summary>
      [STAThread]
      static void Main() {
         Application.EnableVisualStyles();
         Application.SetCompatibleTextRenderingDefault(false);
         Application.Run(new frmMain());
      }
   }
}