Mustapha Alami Mohamed Bekkali Latifa Faouzi Driss Zhani Free poset algebras and combinatorics of cones. 15-26 2007 49 Ann. Math. Artif. Intell. 1-4 http://dx.doi.org/10.1007/s10472-007-9058-1 db/journals/amai/amai49.html#AlamiBFZ07