There are two types of feature template: a) the source-side bag-of-word (BOW) features; and b) the target-side order-aware BOW features.
given a context x, y <t , we extract features according to the above templates:  Figure 8: (a) The original hierarchical clustering tree, with left branching to have all leaves at the same tree depth; (b) The hierarchical clustering tree with depth 2, and a virtual task is constructed at depth 3.