1. Faculty of Applied Sciences, Macao Polytechnic University, R. de Luís Gonzaga Gomes, Macao SAR 999078, 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 SAR 999078, 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