Instead of collapsing all non-terminals in the source language into a single symbol X as in Chiang (2007), given a word sequence f i j from position i to position j, we first find heads and then concatenate the POS tags of these heads as f i j 's non-terminal symbol.
we adopt unlabeled dependency structure to derive heads, which are defined as: is regarded as a head if it is dominated by a word outside of this sequence.