@ARTICLE{DeFrutosEtAl08b, title = {Ready to preorder: an algebraic and general proof}, author = {de Frutos Escrig, David and Carlos Gregorio-Rodr{\'\i}guez and Miguel Palomino}, journal = {Journal of Logic and Algebraic Programming}, volume = 78, number = 7, pages = {539-551}, year = 2009 }