I have specific question about suffix trees. I am reading the book Algorithms on strings_trees and sequence. I cannot understand details of Ukkonen's algorithm for constructing suffix trees. Why using the skip/count trick, any phase of Ukkonen's algorithm takes O(m) time (theorem 6.1.1)? In particular why over the entire phase, the total number of edge traversals during down-walks is bounded by by 3m?