An arrow points from a box at left labeled “root” to a three-tiered box that is at the top of three rows of boxes. The top tier contains “40”; an arrow points from the second tier to a box in the second row (left) with “20” in the top tier; an arrow points from the bottom tier to a box in the second row (right) with “50” and “NULL” in the first and second tiers, respectively. An arrow points from the second tier of the “20” box down to a box in the third row (left) with “10”, “NULL”, and “NULL” in the three tiers, respectively. An arrow points from the bottom tier of the “20” box down to a box in the third row (center) with “30”, “NULL”, and “NULL” in the three tiers, respectively. An arrow points from the bottom tier of the “50” box in the second row down to a box in the third row (right) with “60”, “NULL”, and “NULL” in the three tiers, respectively.