Lookahead Skip-node The second approximation, Lookahead Skip-node, is related to the observation that when growing a substructure, we do not have to confine the growth only towards ancestors, as DFS traversal already ensures iterative manner of computation.
the constraint v i is an ancestor of v k can be dropped: In addition to those features generated by Linear Skip-node in Figure 5(a), Lookahead Skip-node can generate additional tree substructures, shown in Figure 5(b).