However, this simple mapping may be ineffective since the trees within a pair, e.g., (t 1 , r), are treated independently, and no meaningful features connecting t 1 and r can be derived from their tree fragments.
we model φ M (r, t 1 ) by using word-matching relations between t 1 and r, such that connections between words and constituents of the two trees are established using position-independent word matching.