For example, the Alignment feature in Figure 2(a) is local, and thus can be computed a priori, but the Word Trigrams is not; in Figure 2(b) words in parentheses are subgenerations created so far at each word node; their combination gives rise to the trigrams serving as input to the feature.
this combination may not take place at their immediate ancestors, since these may not be adjacent nodes in the hypergraph.