http://www.google.es/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=8&cad=rja&ved=0CGQQFjAH&url=http%3A%2F%2Fwww.unicaen.fr%2Fpuc%2Fecrire%2Frevues%2Fkentron%2Fkentron20%2Fk2008boyaval.pdf&ei=jEsiUZLKNMHM0QXYxIGAAQ&usg=AFQjCNGQwcFKoHIqZQvqK7773LNmKaMfaw&sig