@article{JAC_2000__12_1_a6,
     author = {Billera, Louis J. and Liu, Niandong},
     title = {Noncommutative enumeration in graded posets},
     journal = {Journal of Algebraic Combinatorics},
     pages = {7--24},
     publisher = {mathdoc},
     volume = {12},
     number = {1},
     year = {2000},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/JAC_2000__12_1_a6/}
}