To improve the speed computation of T K, we consider pairs of nodes (n 1 , n 2 ) belonging to the same tree level.
given H, the height of the STRUCT trees, where each level h contains nodes of the same type, i.e., chunk, POS, and lexical nodes, we define SHTK as the following 5 : where N h T 1 and N h T 2 are sets of nodes at height h. The above equation can be applied with any ∆ function.