public static void TestItr <AnyType>(string type, TreeIterator <AnyType> itr)
 {
     try
     {
         Console.WriteLine(type + ":");
         for (itr.First( ); itr.IsValid( ); itr.Advance( ))
         {
             Console.Write(" " + itr.Retrieve( ));
         }
         Console.WriteLine( );
         itr.Advance( );
     }
     catch (IndexOutOfRangeException e)
     { Console.WriteLine(e + " (as expected)"); }
 }