(correction of the example) (105: ) (106: A) (107: X,B) (108: C,D) (109: E ) (110: ) moving down X will result in either (105: A) (106: B) (107: X ) (108: C,D) (109: E ) (110: ) or equivalently (105: ) (106: A) (107: B ) (108: X ) (109: C,D) (110: E)