An arrow points from a box at left labeled “front” to the top box in a column of three three-tiered boxes. The middle tier of these boxes contain “1”, “2”, and “3”, respectively. The top tier of the “1” box and the bottom tier of the “3” box are empty. An arrow points from the bottom tier of the “1” box to the “2” box, and from the bottom tier of the “2” box to the “3” box; an arrow points from the top tier of the “2” box up to the “1” box, and from the top tier of the “3” box up to the “2” box. An arrow points from a box at left labeled “back” to the “3” box to its right.