1. Departamento de Matemática Aplicada, ETSIAE, Universidad Politécnica de Madrid, Plaza Cardenal Cisneros, 28040 Madrid, Spain
2. Cognitive and Information Sciences, University of California Merced, 5200 North Lake Road Merced, 95343 CA, USA
3. School of Mathematical Sciences, Queen Mary University of London, Mile End Road, E1 4NS London, UK
4. Complexity and Quantitative Linguistics Lab, Laboratory for Relational Algorithmics, Complexity and Learning (LARCA), Institut de Ciències de l’Educació; Universitat Politècnica de Catalunya, Barcelona, Spain