1. a LSV, ENS Cachan, CNRS, INRIA , 61 av. Pdt. Wilson, 94235 , Cachan Cedex, France
2. b Informatics and Mathematical Modelling , Technical University of Denmark , Richard Petersens Plads, DK-2800 , Kgs. Lyngby , Denmark
3. c Department of Mathematics , University of Johannesburg , Private Bag X1, Auckland Park, 2006 , South Africa