foo1.cs

    private void ModifyItemsCount(int newCount)
    {
      if (myTail == null || myTail is FrugalLocalMarkerList<T>)
      {
        if (newCount == 0)
          myTail = null;
        else if (newCount == 1)
          myTail = FrugalLocalMarkerList<T>.One;
        else if (newCount == 2)
          myTail = FrugalLocalMarkerList<T>.Two;
        else
          myTail = new FrugalLocalTailList<T>(newCount);
      }
      else
      {
        myTail.ItemsCount = newCount;
      }
    }

foo2.cs

    private void ModifyItemsCount(int newCount)
    {
      if (myTail == null || myTail is FrugalLocalMarkerList<T>)
      {
        myTail = newCount switch {
          0 => null,
          1 => FrugalLocalMarkerList<T>.One,
          2 => FrugalLocalMarkerList<T>.Two,
          _ => new FrugalLocalTailList<T>(newCount)
        };
      }
      else
      {
        myTail.ItemsCount = newCount;
      }
    }