For example, to translate the Chinese phrase " ", we can extract a rule (R2 in Figure 9) directly from the U-tree because the phrase " " is governed by a frontier node, i.e., node "VBD+RB".
because no node governs " " in the binary parse tree, we can only obtain a rule (R1 in Figure 9) with many extra nodes and edges, such as node CD in R1.