1. Faculty of Applied Sciences, Macao Polytechnic University, R. de Luís Gonzaga Gomes, Macao, China
2. Engineering Research Centre of Applied Technology on Machine Translation and Artificial Intelligence of Ministry of Education, Macao Polytechnic University, R. de Luís Gonzaga Gomes, Macao, China
3. Department of Computer Science and Engineering—DISI, University of Bologna, Via Zamboni, 33, 40126 Bologna, Italy
4. Computer Science Department, UCLA, 404 Westwood Plaza, Westwood, Los Angeles, CA 90095-1596, USA