1. Department of Informatics, King's College London, London WC2R 2LS, UK
2. Laboratoire d'informatique Gaspard-Monge, Université Paris-Est, Paris, France
3. Department of Mathematics and Informatics, University of Palermo, Palermo, Italy
4. AℓEDA Group, Department of CSE, BUET, Dhaka-1000, Bangladesh