using System;
using System.Collections.Generic;
using System.Linq;
 
public class Test
{
  private static void DoIt(IEnumerable<int> a)
  {
    Console.WriteLine(String.Join(" ", a));
 
    foreach (var x in a.Zip(a.Skip(1), (x, y) => Enumerable.Repeat(x, 1).Concat(Enumerable.Repeat(y, 1))).Zip(a.Skip(2), (xy, z) => xy.Concat(Enumerable.Repeat(z, 1))).Where((x, i) => i % 3 == 0))
      Console.WriteLine(String.Join(" ", x));
 
    Console.WriteLine();
  }
 
  public static void Main()
  {
    DoIt(new int[] {1});
    DoIt(new int[] {1, 2});
    DoIt(new int[] {1, 2, 3});
    DoIt(new int[] {1, 2, 3, 4});
    DoIt(new int[] {1, 2, 3, 4, 5});
    DoIt(new int[] {1, 2, 3, 4, 5, 6});
  }
}

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