Hammerton (2003) was the first to exploit LSTM for sequence labeling. built a BiLSTM-CRF structure, which has been extended by adding character-level LSTM (Lample et al., 2016; <cite>Liu et al., 2018)</cite> , GRU (Yang et al., 2016) , and CNN (Chiu and Nichols, 2016; Ma and Hovy, 2016) features.