If o i is positive, then the head of w i is the o i -th token to the right that has the part-of-speech tag p i .
we denote LSTM θ (w) as the abstraction of a long short-term memory network, that processes an input sentence w=[w 1 , ..., w n ] to generate an output of hidden representations a BILSTM can be seen as BILSTM In this work we will stack two layers of BILSTMs before decoding the output.