1. Department of Computing, Macquarie University, Australia. E-mail: dat.nguyen@students.mq.edu.au
2. Department of Computational Linguistics, Saarland University, Germany. E-mail: daiquocn@coli.uni-saarland.de
3. VNU University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam. E-mail: sonpb@vnu.edu.vn