@article{EXMATH_1994_1994_1_a16,
     author = {Bujosa, Andr\'es and Criado, Regino},
     title = {Un anillo para describir expresiones simb\'olicas.},
     journal = {Extracta mathematicae},
     pages = {206-212},
     year = {1994},
     volume = {9},
     number = {3},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EXMATH_1994_1994_1_a16/}
}